We have some Dockerfiles that reference Ubuntu 19.04, which is not an LTS
version and has now appears to have been retired from the Ubuntu repositories.
Switch to 18.04, which is the current long-term support version. This
also requires a switch from OpenJDK 12 to 11.
Also change a usage of 16.04 to 18.04, for consistency.
We have some Dockerfiles that reference Ubuntu 19.04, which is not an LTS
version and has now appears to have been retired from the Ubuntu repositories.
Switch to 18.04, which is the current long-term support version. Also change a
usage of 16.04 to 18.04, for consistency.
Similarly to what has be done for Azure in #48636, this commit
adds a new :test:fixtures:gcs-fixture project which provides two
docker-compose based fixtures that emulate a Google Cloud
Storage service.
Some code has been extracted from existing tests and placed
into this new project so that it can be easily reused in other
projects.