Support Singularity models in registry

[hans]

Currently the public registry only supports Docker models. Ideally we'd have multiple images per model, some of which may be on the Docker platform and some of which may be on the Singularity platform.

But I'll keep this marked as low-priority until we have a user with a clear need. For our own purposes, I'll be building Singularity images locally from the registry Docker images and just using those.