Create cross-platform docker images in github

[baentsch]

• Become independent from CircleCI for docker image creation
• Create multi-architecture images
• Publish those to Github Packages and Docker Hub
• Automate release tagging for docker images

[baentsch]

First implementation in #214. Feedback welcome which platforms to activate and which apps to roll over to github CI.

Shall we set a goal to completely replace CircleCI? @dstebila @christianpaquin ?

Even more radical thought: Shall we work on replacing Travis and AppVeyor, too? This way we might do/become really good in one CI technology instead of "dabbling" in several...

[dstebila]

GitHub Actions does seem to be smoother running, so I don't mind if we migrate. (Although aren't there are a few things where we need to stay with CircleCI, e.g. the PowerPC s390 testing that Basil set up?) I also don't feel sufficiently strongly to make a huge push for a migration, so to me it would be an as-we-feel-like-it + new-things-go-on-GitHub-by-default migration.