chore(ci): build multiarch in github actions

[jeboehm]

Hi I deploy Dockerize by using multistage Docker builds and copying the binary from your provided Docker image very often. Yet there are no arm64 images available although ARM on Mac and ARM-Servers are more and more common.

This pull request adds a a workflow to build and push images to the ghcr.io repository for both architectures. Additionally, the images will be signed with the commit hash to provide a little extra layer of security (I think).

It was not possible for me to find out how you push images to the docker.io repository at the moment. I'd suggest to extend the workflow to do that: https://docs.docker.com/build/ci/github-actions/push-multi-registries/