Adding support for arm64 architecture.

[ywoelker]

With the new Apple M1 chips (arm64 architecture) the conversion of Docker images to Singularity images with this container needs twice as much time. I tried to add support for building this Docker image with the amd64 and arm64 architecture and upload both of them to Docker Hub so no one has to care and can pull the image with the matching architecture.

[kathoef]

Thank you a lot! It took a while, but today I finally had the time to look into your PR. It seems, there were a bit more Github actions related things to add in order to make the arm64 build work.

Since at the moment, unfortunately, I don't have time to work on such projects together, I took the shortcut and implemented further required changes in #4 myself. (This also includes changes to the Go binary architecture and to Singularity itself, which is now maintained as Apptainer, and was not installing anymore from the URL that was specified in the Dockerfile.)

I would be very happy, if you could test the arm64 image that will be available on Dockerhub when the currently running workflow succeeded, and if you could provide a PR with container build example commands for the README.md? Of particular interest: How to build and convert amd64 container images on arm64 machines, which is, I suppose, what should be done for running the Singularity image on the target HPC system architecture? @willirath mentioned you did a few tests and having details about how to do this would be very helpful!