Re-use and tag master CI test image for merge requests

For MRs we only build the branch-specific CI test container image if changes are pushed to Dockerfile or requirements.txt, because the build takes a few minutes. When starting a new MR it would be good to use master CI test image tag when no new build is required.

We could try create a new job which pulls the :master tag, and pushed it back with a tag using the branch name, in the case we don't need to build a new image. This will save a few minutes.