Check/fixup/extend support for building from other docker registries

[yarikoptic]

ATM we have all those generate_singularity_for_docker_image (still "hardcoded" in the code), see https://github.com/ReproNim/containers/blob/master/scripts/create_singularities#L545 , which fetch/build packages from docker hub. We should extend that support for ghcr, in particular immediate target

• enh: add skullduggery #159

after tagged release added