Support for multi-platform docker containers

[sjunges]

See https://docs.docker.com/build/building/multi-platform/, which avoids QEMU

[volkm]

My proposal is to use multiple runners in Github actions to build the different platform images. The build would most likely be slow for the emulated runners, but at least we get multi-platform images. If arm64 runners are available in the CI at some point (probably end of the year), we could easily switch to a native build.

Another possibility in the meantime could be to build the amd64 images via the CI and manually build arm64 on our Macbook. Then we could manually merge both image types into one.