The Dockerfile is still using `apk` instead of `apt-get` (Debian slim).
So that's why our build server was never successfully pushing the
`maintenance-worker` image.
Inspired by the tagging of e.g. `node` on dockerhub:
https://hub.docker.com/_/node/?tab=description
I would like to push the current version of all our images to dockerhub.
This is a first step to push to production later on.
If sb. increments the VERSION file, this is considered a release.