Use docker-builder

Use docker-builder to build the docker image.

This change also modifies when the images are tested. They're built with the commit hash as a tag, then they're tested, and if they pass the tests, then they get tagged with different names. The current versions tags first and tests later, which can result in broken tags.

It seems it's not possible to use $HARBOR_* variables on their own in the variables section of the CI, which is weird. Luckily, this project group already had variables defined with the correct credentials, so this works.

I also removed the mr_* tag bit because there's another Gitlab bug that apparently stops child pipelines from working in merge requests. 🤦 It may not be needed, though, as there will already be a image for every commit. We could also have a tag for each branch, which would probably be enough to cover the merge request use-case.

Obsoletes !2.