Add tag based multi arch docker build using GitHub Workflows

[TrAnn3l]

Hey,

I added a github workflow which should build multi-arch docker images for every new tag.
I also added the possibility to push the images to ghcr.io (GitHub Packages)
In addition to that, i added a build cache, becuase building images with qemu can quite take a while (around 20 min), so it should be much faster for subsequent builds.

The tagging behaviour should be identically to the current docker hub repo
When a new stable tag is published, e.g. v0.50.1, then the following tags will be created: 0.50.1, 0.50-latest, latest
When a new beta tag is published, e.g. v0.51.1-beta, then the following tags will be created: 0.51.1-beta, 0.51-latest

For the action to work, only the two secrets DOCKER_USERNAME & DOCKER_PASSWORD need to be created and filled in the repo settings. For ghcr.io to work, no secrets are needed, but i think the package visibility needs to be changed after the first build/push.

If this PR gets merged, one would possible need to update the release routine, as everything docker related is now done by GitHub.

You can see my test images here.
Would close #902

[zadam]

Thanks, that looks very useful! I will test it with next release.