command: docker build short: Build an image from a Dockerfile long: |- The `docker build` command builds Docker images from a Dockerfile and a "context". A build's context is the set of files located in the specified `PATH` or `URL`. The build process can refer to any of the files in the context. For example, your build can use a [*COPY*](../builder.md#copy) instruction to reference a file in the context. The `URL` parameter can refer to three kinds of resources: Git repositories, pre-packaged tarball contexts and plain text files. ### Git repositories When the `URL` parameter points to the location of a Git repository, the repository acts as the build context. The system recursively fetches the repository and its submodules. The commit history is not preserved. A repository is first pulled into a temporary directory on your local host. After that succeeds, the directory is sent to the Docker daemon as the context. Local copy gives you the ability to access private repositories using local user credentials, VPN's, and so forth. > **Note** > > If the `URL` parameter contains a fragment the system will recursively clone > the repository and its submodules using a `git clone --recursive` command. Git URLs accept context configuration in their fragment section, separated by a colon (`:`). The first part represents the reference that Git will check out, and can be either a branch, a tag, or a remote reference. The second part represents a subdirectory inside the repository that will be used as a build context. For example, run this command to use a directory called `docker` in the branch `container`: ```bash $ docker build https://github.com/docker/rootfs.git#container:docker ``` The following table represents all the valid suffixes with their build contexts: Build Syntax Suffix | Commit Used | Build Context Used --------------------------------|-----------------------|------------------- `myrepo.git` | `refs/heads/master` | `/` `myrepo.git#mytag` | `refs/tags/mytag` | `/` `myrepo.git#mybranch` | `refs/heads/mybranch` | `/` `myrepo.git#pull/42/head` | `refs/pull/42/head` | `/` `myrepo.git#:myfolder` | `refs/heads/master` | `/myfolder` `myrepo.git#master:myfolder` | `refs/heads/master` | `/myfolder` `myrepo.git#mytag:myfolder` | `refs/tags/mytag` | `/myfolder` `myrepo.git#mybranch:myfolder` | `refs/heads/mybranch` | `/myfolder` > **Note** > > You cannot specify the build-context directory (`myfolder` in the examples above) > when using BuildKit as builder (`DOCKER_BUILDKIT=1`). Support for this feature > is tracked in [buildkit#1684](https://github.com/moby/buildkit/issues/1684). ### Tarball contexts If you pass an URL to a remote tarball, the URL itself is sent to the daemon: ```bash $ docker build http://server/context.tar.gz ``` The download operation will be performed on the host the Docker daemon is running on, which is not necessarily the same host from which the build command is being issued. The Docker daemon will fetch `context.tar.gz` and use it as the build context. Tarball contexts must be tar archives conforming to the standard `tar` UNIX format and can be compressed with any one of the 'xz', 'bzip2', 'gzip' or 'identity' (no compression) formats. ### Text files Instead of specifying a context, you can pass a single `Dockerfile` in the `URL` or pipe the file in via `STDIN`. To pipe a `Dockerfile` from `STDIN`: ```bash $ docker build - < Dockerfile ``` With Powershell on Windows, you can run: ```powershell Get-Content Dockerfile | docker build - ``` If you use `STDIN` or specify a `URL` pointing to a plain text file, the system places the contents into a file called `Dockerfile`, and any `-f`, `--file` option is ignored. In this scenario, there is no context. By default the `docker build` command will look for a `Dockerfile` at the root of the build context. The `-f`, `--file`, option lets you specify the path to an alternative file to use instead. This is useful in cases where the same set of files are used for multiple builds. The path must be to a file within the build context. If a relative path is specified then it is interpreted as relative to the root of the context. In most cases, it's best to put each Dockerfile in an empty directory. Then, add to that directory only the files needed for building the Dockerfile. To increase the build's performance, you can exclude files and directories by adding a `.dockerignore` file to that directory as well. For information on creating one, see the [.dockerignore file](../builder.md#dockerignore-file). If the Docker client loses connection to the daemon, the build is canceled. This happens if you interrupt the Docker client with `CTRL-c` or if the Docker client is killed for any reason. If the build initiated a pull which is still running at the time the build is cancelled, the pull is cancelled as well. usage: docker build [OPTIONS] PATH | URL | - pname: docker plink: docker.yaml options: - option: add-host value_type: list description: Add a custom host-to-IP mapping (host:ip) deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: build-arg value_type: list description: Set build-time variables deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: cache-from value_type: stringSlice default_value: '[]' description: Images to consider as cache sources deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: cgroup-parent value_type: string description: Optional parent cgroup for the container deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: compress value_type: bool default_value: "false" description: Compress the build context using gzip deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: cpu-period value_type: int64 default_value: "0" description: Limit the CPU CFS (Completely Fair Scheduler) period deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: cpu-quota value_type: int64 default_value: "0" description: Limit the CPU CFS (Completely Fair Scheduler) quota deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: cpu-shares shorthand: c value_type: int64 default_value: "0" description: CPU shares (relative weight) deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: cpuset-cpus value_type: string description: CPUs in which to allow execution (0-3, 0,1) deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: cpuset-mems value_type: string description: MEMs in which to allow execution (0-3, 0,1) deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: disable-content-trust value_type: bool default_value: "true" description: Skip image verification deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: file shorthand: f value_type: string description: Name of the Dockerfile (Default is 'PATH/Dockerfile') deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: force-rm value_type: bool default_value: "false" description: Always remove intermediate containers deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: iidfile value_type: string description: Write the image ID to the file deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: isolation value_type: string description: Container isolation technology deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: label value_type: list description: Set metadata for an image deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: memory shorthand: m value_type: bytes default_value: "0" description: Memory limit deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: memory-swap value_type: bytes default_value: "0" description: | Swap limit equal to memory plus swap: '-1' to enable unlimited swap deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: network value_type: string default_value: default description: | Set the networking mode for the RUN instructions during build deprecated: false min_api_version: "1.25" experimental: false experimentalcli: false kubernetes: false swarm: false - option: no-cache value_type: bool default_value: "false" description: Do not use cache when building the image deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: output shorthand: o value_type: stringArray default_value: '[]' description: 'Output destination (format: type=local,dest=path)' deprecated: false min_api_version: "1.40" experimental: false experimentalcli: false kubernetes: false swarm: false - option: platform value_type: string description: Set platform if server is multi-platform capable deprecated: false min_api_version: "1.38" experimental: false experimentalcli: false kubernetes: false swarm: false - option: progress value_type: string default_value: auto description: | Set type of progress output (auto, plain, tty). Use plain to show container output deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: pull value_type: bool default_value: "false" description: Always attempt to pull a newer version of the image deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: quiet shorthand: q value_type: bool default_value: "false" description: Suppress the build output and print image ID on success deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: rm value_type: bool default_value: "true" description: Remove intermediate containers after a successful build deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: secret value_type: stringArray default_value: '[]' description: | Secret file to expose to the build (only if BuildKit enabled): id=mysecret,src=/local/secret deprecated: false min_api_version: "1.39" experimental: false experimentalcli: false kubernetes: false swarm: false - option: security-opt value_type: stringSlice default_value: '[]' description: Security options deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: shm-size value_type: bytes default_value: "0" description: Size of /dev/shm deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: squash value_type: bool default_value: "false" description: Squash newly built layers into a single new layer deprecated: false min_api_version: "1.25" experimental: true experimentalcli: false kubernetes: false swarm: false - option: ssh value_type: stringArray default_value: '[]' description: | SSH agent socket or keys to expose to the build (only if BuildKit enabled) (format: default|[=|[,]]) deprecated: false min_api_version: "1.39" experimental: false experimentalcli: false kubernetes: false swarm: false - option: stream value_type: bool default_value: "false" description: Stream attaches to server to negotiate build context deprecated: false min_api_version: "1.31" experimental: true experimentalcli: false kubernetes: false swarm: false - option: tag shorthand: t value_type: list description: Name and optionally a tag in the 'name:tag' format deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: target value_type: string description: Set the target build stage to build. deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false - option: ulimit value_type: ulimit default_value: '[]' description: Ulimit options deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false examples: |- ### Build with PATH ```bash $ docker build . Uploading context 10240 bytes Step 1/3 : FROM busybox Pulling repository busybox ---> e9aa60c60128MB/2.284 MB (100%) endpoint: https://cdn-registry-1.docker.io/v1/ Step 2/3 : RUN ls -lh / ---> Running in 9c9e81692ae9 total 24 drwxr-xr-x 2 root root 4.0K Mar 12 2013 bin drwxr-xr-x 5 root root 4.0K Oct 19 00:19 dev drwxr-xr-x 2 root root 4.0K Oct 19 00:19 etc drwxr-xr-x 2 root root 4.0K Nov 15 23:34 lib lrwxrwxrwx 1 root root 3 Mar 12 2013 lib64 -> lib dr-xr-xr-x 116 root root 0 Nov 15 23:34 proc lrwxrwxrwx 1 root root 3 Mar 12 2013 sbin -> bin dr-xr-xr-x 13 root root 0 Nov 15 23:34 sys drwxr-xr-x 2 root root 4.0K Mar 12 2013 tmp drwxr-xr-x 2 root root 4.0K Nov 15 23:34 usr ---> b35f4035db3f Step 3/3 : CMD echo Hello world ---> Running in 02071fceb21b ---> f52f38b7823e Successfully built f52f38b7823e Removing intermediate container 9c9e81692ae9 Removing intermediate container 02071fceb21b ``` This example specifies that the `PATH` is `.`, and so all the files in the local directory get `tar`d and sent to the Docker daemon. The `PATH` specifies where to find the files for the "context" of the build on the Docker daemon. Remember that the daemon could be running on a remote machine and that no parsing of the Dockerfile happens at the client side (where you're running `docker build`). That means that *all* the files at `PATH` get sent, not just the ones listed to [*ADD*](../builder.md#add) in the Dockerfile. The transfer of context from the local machine to the Docker daemon is what the `docker` client means when you see the "Sending build context" message. If you wish to keep the intermediate containers after the build is complete, you must use `--rm=false`. This does not affect the build cache. ### Build with URL ```bash $ docker build github.com/creack/docker-firefox ``` This will clone the GitHub repository and use the cloned repository as context. The Dockerfile at the root of the repository is used as Dockerfile. You can specify an arbitrary Git repository by using the `git://` or `git@` scheme. ```bash $ docker build -f ctx/Dockerfile http://server/ctx.tar.gz Downloading context: http://server/ctx.tar.gz [===================>] 240 B/240 B Step 1/3 : FROM busybox ---> 8c2e06607696 Step 2/3 : ADD ctx/container.cfg / ---> e7829950cee3 Removing intermediate container b35224abf821 Step 3/3 : CMD /bin/ls ---> Running in fbc63d321d73 ---> 3286931702ad Removing intermediate container fbc63d321d73 Successfully built 377c409b35e4 ``` This sends the URL `http://server/ctx.tar.gz` to the Docker daemon, which downloads and extracts the referenced tarball. The `-f ctx/Dockerfile` parameter specifies a path inside `ctx.tar.gz` to the `Dockerfile` that is used to build the image. Any `ADD` commands in that `Dockerfile` that refers to local paths must be relative to the root of the contents inside `ctx.tar.gz`. In the example above, the tarball contains a directory `ctx/`, so the `ADD ctx/container.cfg /` operation works as expected. ### Build with - ```bash $ docker build - < Dockerfile ``` This will read a Dockerfile from `STDIN` without context. Due to the lack of a context, no contents of any local directory will be sent to the Docker daemon. Since there is no context, a Dockerfile `ADD` only works if it refers to a remote URL. ```bash $ docker build - < context.tar.gz ``` This will build an image for a compressed context read from `STDIN`. Supported formats are: bzip2, gzip and xz. ### Use a .dockerignore file ```bash $ docker build . Uploading context 18.829 MB Uploading context Step 1/2 : FROM busybox ---> 769b9341d937 Step 2/2 : CMD echo Hello world ---> Using cache ---> 99cc1ad10469 Successfully built 99cc1ad10469 $ echo ".git" > .dockerignore $ docker build . Uploading context 6.76 MB Uploading context Step 1/2 : FROM busybox ---> 769b9341d937 Step 2/2 : CMD echo Hello world ---> Using cache ---> 99cc1ad10469 Successfully built 99cc1ad10469 ``` This example shows the use of the `.dockerignore` file to exclude the `.git` directory from the context. Its effect can be seen in the changed size of the uploaded context. The builder reference contains detailed information on [creating a .dockerignore file](../builder.md#dockerignore-file). When using the [BuildKit backend](../builder.md#buildkit), `docker build` searches for a `.dockerignore` file relative to the Dockerfile name. For example, running `docker build -f myapp.Dockerfile .` will first look for an ignore file named `myapp.Dockerfile.dockerignore`. If such a file is not found, the `.dockerignore` file is used if present. Using a Dockerfile based `.dockerignore` is useful if a project contains multiple Dockerfiles that expect to ignore different sets of files. ### Tag an image (-t) ```bash $ docker build -t vieux/apache:2.0 . ``` This will build like the previous example, but it will then tag the resulting image. The repository name will be `vieux/apache` and the tag will be `2.0`. [Read more about valid tags](tag.md). You can apply multiple tags to an image. For example, you can apply the `latest` tag to a newly built image and add another tag that references a specific version. For example, to tag an image both as `whenry/fedora-jboss:latest` and `whenry/fedora-jboss:v2.1`, use the following: ```bash $ docker build -t whenry/fedora-jboss:latest -t whenry/fedora-jboss:v2.1 . ``` ### Specify a Dockerfile (-f) ```bash $ docker build -f Dockerfile.debug . ``` This will use a file called `Dockerfile.debug` for the build instructions instead of `Dockerfile`. ```bash $ curl example.com/remote/Dockerfile | docker build -f - . ``` The above command will use the current directory as the build context and read a Dockerfile from stdin. ```bash $ docker build -f dockerfiles/Dockerfile.debug -t myapp_debug . $ docker build -f dockerfiles/Dockerfile.prod -t myapp_prod . ``` The above commands will build the current build context (as specified by the `.`) twice, once using a debug version of a `Dockerfile` and once using a production version. ```bash $ cd /home/me/myapp/some/dir/really/deep $ docker build -f /home/me/myapp/dockerfiles/debug /home/me/myapp $ docker build -f ../../../../dockerfiles/debug /home/me/myapp ``` These two `docker build` commands do the exact same thing. They both use the contents of the `debug` file instead of looking for a `Dockerfile` and will use `/home/me/myapp` as the root of the build context. Note that `debug` is in the directory structure of the build context, regardless of how you refer to it on the command line. > **Note** > > `docker build` returns a `no such file or directory` error if the > file or directory does not exist in the uploaded context. This may > happen if there is no context, or if you specify a file that is > elsewhere on the Host system. The context is limited to the current > directory (and its children) for security reasons, and to ensure > repeatable builds on remote Docker hosts. This is also the reason why > `ADD ../file` does not work. ### Use a custom parent cgroup (--cgroup-parent) When `docker build` is run with the `--cgroup-parent` option the containers used in the build will be run with the [corresponding `docker run` flag](../run.md#specify-custom-cgroups). ### Set ulimits in container (--ulimit) Using the `--ulimit` option with `docker build` will cause each build step's container to be started using those [`--ulimit` flag values](run.md#set-ulimits-in-container---ulimit). ### Set build-time variables (--build-arg) You can use `ENV` instructions in a Dockerfile to define variable values. These values persist in the built image. However, often persistence is not what you want. Users want to specify variables differently depending on which host they build an image on. A good example is `http_proxy` or source versions for pulling intermediate files. The `ARG` instruction lets Dockerfile authors define values that users can set at build-time using the `--build-arg` flag: ```bash $ docker build --build-arg HTTP_PROXY=http://10.20.30.2:1234 --build-arg FTP_PROXY=http://40.50.60.5:4567 . ``` This flag allows you to pass the build-time variables that are accessed like regular environment variables in the `RUN` instruction of the Dockerfile. Also, these values don't persist in the intermediate or final images like `ENV` values do. You must add `--build-arg` for each build argument. Using this flag will not alter the output you see when the `ARG` lines from the Dockerfile are echoed during the build process. For detailed information on using `ARG` and `ENV` instructions, see the [Dockerfile reference](../builder.md). You may also use the `--build-arg` flag without a value, in which case the value from the local environment will be propagated into the Docker container being built: ```bash $ export HTTP_PROXY=http://10.20.30.2:1234 $ docker build --build-arg HTTP_PROXY . ``` This is similar to how `docker run -e` works. Refer to the [`docker run` documentation](https://docs.docker.com/engine/reference/commandline/run/#set-environment-variables--e---env---env-file) for more information. ### Optional security options (--security-opt) This flag is only supported on a daemon running on Windows, and only supports the `credentialspec` option. The `credentialspec` must be in the format `file://spec.txt` or `registry://keyname`. ### Specify isolation technology for container (--isolation) This option is useful in situations where you are running Docker containers on Windows. The `--isolation=` option sets a container's isolation technology. On Linux, the only supported is the `default` option which uses Linux namespaces. On Microsoft Windows, you can specify these values: | Value | Description | |-----------|---------------------------------------------------------------------------------------------------------------------------------------------------------------| | `default` | Use the value specified by the Docker daemon's `--exec-opt` . If the `daemon` does not specify an isolation technology, Microsoft Windows uses `process` as its default value. | | `process` | Namespace isolation only. | | `hyperv` | Hyper-V hypervisor partition-based isolation. | Specifying the `--isolation` flag without a value is the same as setting `--isolation="default"`. ### Add entries to container hosts file (--add-host) You can add other hosts into a container's `/etc/hosts` file by using one or more `--add-host` flags. This example adds a static address for a host named `docker`: $ docker build --add-host=docker:10.180.0.1 . ### Specifying target build stage (--target) When building a Dockerfile with multiple build stages, `--target` can be used to specify an intermediate build stage by name as a final stage for the resulting image. Commands after the target stage will be skipped. ```dockerfile FROM debian AS build-env ... FROM alpine AS production-env ... ``` ```bash $ docker build -t mybuildimage --target build-env . ``` ### Custom build outputs By default, a local container image is created from the build result. The `--output` (or `-o`) flag allows you to override this behavior, and a specify a custom exporter. For example, custom exporters allow you to export the build artifacts as files on the local filesystem instead of a Docker image, which can be useful for generating local binaries, code generation etc. The value for `--output` is a CSV-formatted string defining the exporter type and options. Currently, `local` and `tar` exporters are supported. The `local` exporter writes the resulting build files to a directory on the client side. The `tar` exporter is similar but writes the files as a single tarball (`.tar`). If no type is specified, the value defaults to the output directory of the local exporter. Use a hyphen (`-`) to write the output tarball to standard output (`STDOUT`). The following example builds an image using the current directory (`.`) as build context, and exports the files to a directory named `out` in the current directory. If the directory does not exist, Docker creates the directory automatically: ```bash $ docker build -o out . ``` The example above uses the short-hand syntax, omitting the `type` options, and thus uses the default (`local`) exporter. The example below shows the equivalent using the long-hand CSV syntax, specifying both `type` and `dest` (destination path): ```bash $ docker build --output type=local,dest=out . ``` Use the `tar` type to export the files as a `.tar` archive: ```bash $ docker build --output type=tar,dest=out.tar . ``` The example below shows the equivalent when using the short-hand syntax. In this case, `-` is specified as destination, which automatically selects the `tar` type, and writes the output tarball to standard output, which is then redirected to the `out.tar` file: ```bash docker build -o - . > out.tar ``` The `--output` option exports all files from the target stage. A common pattern for exporting only specific files is to do multi-stage builds and to copy the desired files to a new scratch stage with [`COPY --from`](../builder.md#copy). The example `Dockerfile` below uses a separate stage to collect the build-artifacts for exporting: ```dockerfile FROM golang AS build-stage RUN go get -u github.com/LK4D4/vndr FROM scratch AS export-stage COPY --from=build-stage /go/bin/vndr / ``` When building the Dockerfile with the `-o` option, only the files from the final stage are exported to the `out` directory, in this case, the `vndr` binary: ```bash $ docker build -o out . [+] Building 2.3s (7/7) FINISHED => [internal] load build definition from Dockerfile 0.1s => => transferring dockerfile: 176B 0.0s => [internal] load .dockerignore 0.0s => => transferring context: 2B 0.0s => [internal] load metadata for docker.io/library/golang:latest 1.6s => [build-stage 1/2] FROM docker.io/library/golang@sha256:2df96417dca0561bf1027742dcc5b446a18957cd28eba6aa79269f23f1846d3f 0.0s => => resolve docker.io/library/golang@sha256:2df96417dca0561bf1027742dcc5b446a18957cd28eba6aa79269f23f1846d3f 0.0s => CACHED [build-stage 2/2] RUN go get -u github.com/LK4D4/vndr 0.0s => [export-stage 1/1] COPY --from=build-stage /go/bin/vndr / 0.2s => exporting to client 0.4s => => copying files 10.30MB 0.3s $ ls ./out vndr ``` > **Note** > > This feature requires the BuildKit backend. You can either > [enable BuildKit](../builder.md#buildkit) or use the [buildx](https://github.com/docker/buildx) > plugin which provides more output type options. ### Specifying external cache sources In addition to local build cache, the builder can reuse the cache generated from previous builds with the `--cache-from` flag pointing to an image in the registry. To use an image as a cache source, cache metadata needs to be written into the image on creation. This can be done by setting `--build-arg BUILDKIT_INLINE_CACHE=1` when building the image. After that, the built image can be used as a cache source for subsequent builds. Upon importing the cache, the builder will only pull the JSON metadata from the registry and determine possible cache hits based on that information. If there is a cache hit, the matched layers are pulled into the local environment. In addition to images, the cache can also be pulled from special cache manifests generated by [`buildx`](https://github.com/docker/buildx) or the BuildKit CLI (`buildctl`). These manifests (when built with the `type=registry` and `mode=max` options) allow pulling layer data for intermediate stages in multi-stage builds. The following example builds an image with inline-cache metadata and pushes it to a registry, then uses the image as a cache source on another machine: ```bash $ docker build -t myname/myapp --build-arg BUILDKIT_INLINE_CACHE=1 . $ docker push myname/myapp ``` After pushing the image, the image is used as cache source on another machine. BuildKit automatically pulls the image from the registry if needed. ```bash # on another machine $ docker build --cache-from myname/myapp . ``` > **Note** > > This feature requires the BuildKit backend. You can either > [enable BuildKit](../builder.md#buildkit) or use the [buildx](https://github.com/docker/buildx) > plugin. The previous builder has limited support for reusing cache from > pre-pulled images. ### Squash an image's layers (--squash) (experimental) #### Overview Once the image is built, squash the new layers into a new image with a single new layer. Squashing does not destroy any existing image, rather it creates a new image with the content of the squashed layers. This effectively makes it look like all `Dockerfile` commands were created with a single layer. The build cache is preserved with this method. The `--squash` option is an experimental feature, and should not be considered stable. Squashing layers can be beneficial if your Dockerfile produces multiple layers modifying the same files, for example, files that are created in one step, and removed in another step. For other use-cases, squashing images may actually have a negative impact on performance; when pulling an image consisting of multiple layers, layers can be pulled in parallel, and allows sharing layers between images (saving space). For most use cases, multi-stage builds are a better alternative, as they give more fine-grained control over your build, and can take advantage of future optimizations in the builder. Refer to the [use multi-stage builds](https://docs.docker.com/develop/develop-images/multistage-build/) section in the userguide for more information. #### Known limitations The `--squash` option has a number of known limitations: - When squashing layers, the resulting image cannot take advantage of layer sharing with other images, and may use significantly more space. Sharing the base image is still supported. - When using this option you may see significantly more space used due to storing two copies of the image, one for the build cache with all the cache layers in tact, and one for the squashed version. - While squashing layers may produce smaller images, it may have a negative impact on performance, as a single layer takes longer to extract, and downloading a single layer cannot be parallelized. - When attempting to squash an image that does not make changes to the filesystem (for example, the Dockerfile only contains `ENV` instructions), the squash step will fail (see [issue #33823](https://github.com/moby/moby/issues/33823)). #### Prerequisites The example on this page is using experimental mode in Docker 19.03. Experimental mode can be enabled by using the `--experimental` flag when starting the Docker daemon or setting `experimental: true` in the `daemon.json` configuration file. By default, experimental mode is disabled. To see the current configuration of the docker daemon, use the `docker version` command and check the `Experimental` line in the `Engine` section: ```console Client: Docker Engine - Community Version: 19.03.8 API version: 1.40 Go version: go1.12.17 Git commit: afacb8b Built: Wed Mar 11 01:21:11 2020 OS/Arch: darwin/amd64 Experimental: false Server: Docker Engine - Community Engine: Version: 19.03.8 API version: 1.40 (minimum version 1.12) Go version: go1.12.17 Git commit: afacb8b Built: Wed Mar 11 01:29:16 2020 OS/Arch: linux/amd64 Experimental: true [...] ``` To enable experimental mode, users need to restart the docker daemon with the experimental flag enabled. #### Enable Docker experimental Experimental features are now included in the standard Docker binaries as of version 1.13.0. For enabling experimental features, you need to start the Docker daemon with `--experimental` flag. You can also enable the daemon flag via `/etc/docker/daemon.json`. e.g. ```json { "experimental": true } ``` Then make sure the experimental flag is enabled: ```bash $ docker version -f '{{.Server.Experimental}}' true ``` #### Build an image with `--squash` argument The following is an example of docker build with `--squash` argument ```dockerfile FROM busybox RUN echo hello > /hello RUN echo world >> /hello RUN touch remove_me /remove_me ENV HELLO=world RUN rm /remove_me ``` An image named `test` is built with `--squash` argument. ```bash $ docker build --squash -t test . [...] ``` If everything is right, the history looks like this: ```bash $ docker history test IMAGE CREATED CREATED BY SIZE COMMENT 4e10cb5b4cac 3 seconds ago 12 B merge sha256:88a7b0112a41826885df0e7072698006ee8f621c6ab99fca7fe9151d7b599702 to sha256:47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb 5 minutes ago /bin/sh -c rm /remove_me 0 B 5 minutes ago /bin/sh -c #(nop) ENV HELLO=world 0 B 5 minutes ago /bin/sh -c touch remove_me /remove_me 0 B 5 minutes ago /bin/sh -c echo world >> /hello 0 B 6 minutes ago /bin/sh -c echo hello > /hello 0 B 7 weeks ago /bin/sh -c #(nop) CMD ["sh"] 0 B 7 weeks ago /bin/sh -c #(nop) ADD file:47ca6e777c36a4cfff 1.113 MB ``` We could find that a layer's name is ``, and there is a new layer with COMMENT `merge`. Test the image, check for `/remove_me` being gone, make sure `hello\nworld` is in `/hello`, make sure the `HELLO` environment variable's value is `world`. deprecated: false experimental: false experimentalcli: false kubernetes: false swarm: false