Add build args: GIT_REPO GIT_REF

[EnigmaCurry]

I think its a good idea to allow building the Docker image from an arbitrary repository and git ref id, otherwise it will always build from your repo at master, which may not be what someone wants. I have set the default ARGS the same as they were before, so this should be identical to the build before if you omit the args.

By the way, my prompt for doing this was that I noticed the latest-arm64 image is much older than the latest (amd64) tag, because I wanted to run the latest version on Raspberry Pi.