WIP: CI: Tag latest schleuder release Docker image with ':release', instead of a specific version

This commit introduces better Docker image tags: By default it will be called 'schleuder:master'. If the git checkout is tagged, it's a release, accordingly, we'll use 'schleuder:release'. This makes it easier for people to track either the master branch or the latest release, and at the same time, makes cleaup of old images obsolete, because we're just overwriting these.

Tested via an intermediate commit to ensure this works as expected.

Closes #318 (closed)