Automate new docker releases

[luckydonald]

Description

It kinda sucks that it takes about another week until a new docker release is available as someone has to bump the version manually and that has to go through a release process.

Especially since it's most of the time just a simple one line change.

I'd rather have it automatically (update that line and) build a container and push that, instead of waiting for people to manually update it. Even if that comes with the risk of some dependency (e.g. apt-get install stuff) being broken as nobody is actually looking at the builds. But if that happens we can always fix it manually again, as we did in the past.