Previously we built a Docker image just for the test,
but the introduction of multi-arch build in 9fbf0952
build also meant the push of the image, so it was
restricted only to the master branch.
This change re-introduces the Docker image build
outside the master branch, which is helpful
in pull requests.
We recently had a few frontend PRs which broke
the Docker image build silently, and that change
prevents it from happening.