Build docker image for multiple processor architectures

[akkie]

This is a proof of concept PR. It currently pushes to the GitHub docker registry. Maybe you can activate the pipeline to see if it works. Later you can then change it to push to docker hub.

I droped the muslrust image in favour of the rust image. This is also noted in the readme: https://github.com/clux/muslrust?tab=readme-ov-file#alternatives

official rust image can target add and easily cross-build when not needing C libraries

I don't think that you need C libs which muslrust provides.

[senpro-ingwersenk]

LGTM!
Took the opportunity (...or excuse :p) to learn a little more about Github workflows. I was dead sure they had native ARM runners...but guess not; there's QEMU and "runs-on" workflows everywhere o.o

[akkie]

I have updated the docker workflow and separated it from the existing release workflow. Also built it based on the official Docker tutorial, because there was an issue with the old workflow where the arch64 images was not published to the registry. It also uses directly the Docker Hub log in now. Only the env variables with the credentials needs to be set.