build(docs-infra): upgrade preview server Docker image to Debian 10 (buster) (#36837)

Previously, the preview server Docker image was based on Debian 9
(stretch).

This commit upgrades the preview server Docker image to Debian 10
(buster) and also upgrades all dependencies to latest versions
(including upgrading Node.js from v10 to v12).

(The GCE VM running the preview server Docker container was also
upgraded from Debian 9 to 10 on 2020-04-27.)

---
Other changes:
- Pinned the installed version of `curl` to make the `aio-health-check`
  and `aio-verify-setup` checks (which use `curl`) more deterministic.
- Dropped the `*-backports` Debian repositories, since they are no
  longer needed. The `*-backports` repositories were introduced in
  593fe5ed25 to install `nginx` from, but
  became obsolete in 2f1a862b83, which
  switched to installing `nginx` from the regular repositories again.
- Added `vim` to the list of installed dependencies (for convenience
  during debugging).

PR Close #36837

aio/aio-builds-setup/dockerbuild/Dockerfile

@@ -1,5 +1,5 @@
 # Image metadata and config
-FROM debian:stretch
+FROM debian:buster
 
 LABEL name="angular.io PR preview" \
       description="This image implements the PR preview functionality for angular.io." \
@@ -37,9 +37,9 @@ ARG TEST_AIO_NGINX_PORT_HTTPS=4433
 ARG      AIO_SIGNIFICANT_FILES_PATTERN='^(?:aio|packages)/(?!.*[._]spec\\.[jt]s$)'
 ARG TEST_AIO_SIGNIFICANT_FILES_PATTERN=$AIO_SIGNIFICANT_FILES_PATTERN
 ARG      AIO_TRUSTED_PR_LABEL="aio: preview"
-ARG TEST_AIO_TRUSTED_PR_LABEL="aio: preview"
+ARG TEST_AIO_TRUSTED_PR_LABEL=$AIO_TRUSTED_PR_LABEL
 ARG      AIO_PREVIEW_SERVER_HOSTNAME=preview.localhost
-ARG TEST_AIO_PREVIEW_SERVER_HOSTNAME=preview.localhost
+ARG TEST_AIO_PREVIEW_SERVER_HOSTNAME=$AIO_PREVIEW_SERVER_HOSTNAME
 ARG      AIO_ARTIFACT_MAX_SIZE=26214400
 ARG TEST_AIO_ARTIFACT_MAX_SIZE=200
 ARG      AIO_PREVIEW_SERVER_PORT=3000
@@ -72,24 +72,24 @@ RUN mkdir /var/log/aio
 
 
 # Add extra package sources
-RUN apt-get update -y && apt-get install -y curl
-RUN curl --silent --show-error --location https://deb.nodesource.com/setup_10.x | bash -
+RUN apt-get update -y && apt-get install -y curl=7.64.0-4+deb10u1
+RUN curl --silent --show-error --location https://deb.nodesource.com/setup_12.x | bash -
 RUN curl --silent --show-error https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
 RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
-RUN echo "deb http://ftp.debian.org/debian stretch-backports main" | tee /etc/apt/sources.list.d/backports.list
 
 
 # Install packages
 RUN apt-get update -y && apt-get install -y \
-    cron=3.0pl1-128+deb9u1 \
-    dnsmasq=2.76-5+deb9u2 \
-    nano=2.7.4-1 \
-    nginx=1.10.3-1+deb9u2 \
-    nodejs=10.15.3-1nodesource1 \
-    openssl=1.1.0j-1~deb9u1 \
-    rsyslog=8.24.0-1 \
-    yarn=1.15.2-1
-RUN yarn global add pm2@3.5.0
+    cron=3.0pl1-134+deb10u1 \
+    dnsmasq=2.80-1 \
+    nano=3.2-3 \
+    nginx=1.14.2-2+deb10u1 \
+    nodejs=12.16.2-1nodesource1 \
+    openssl=1.1.1d-0+deb10u3 \
+    rsyslog=8.1901.0-1 \
+    vim=2:8.1.0875-5 \
+    yarn=1.22.4-1
+RUN yarn global add pm2@4.4.0
 
 
 # Set up log rotation
@@ -162,8 +162,7 @@ RUN find $AIO_SCRIPTS_SH_DIR -maxdepth 1 -type f -printf "%P\n" \
 
 # Set up the Node.js scripts
 COPY scripts-js/ $AIO_SCRIPTS_JS_DIR/
-WORKDIR $AIO_SCRIPTS_JS_DIR/
-RUN yarn install --production --frozen-lockfile
+RUN yarn --cwd="$AIO_SCRIPTS_JS_DIR/" install --production --frozen-lockfile
 
 
 # Set up health check