Support remote registries on self-hosted gitlab instances

[phue]

Hi @vsoch,

First of all, thanks for this great tool!
I'm trying to deploy a custom registry on a self-hosted Gitlab instance and use it on our HPC system.
However, it seems that constructing the url to gitlab pages currently works with the gitlab.io domain only.

singularity-hpc/shpc/main/registry/remote.py

Line 110 in 68a0005

return "https://%s.%s.io/%s/library.json" % (

In my specific case, I would need this pattern: https://%s.pages.%s.rlp.net/%s/library.json instead.
Would it be possible to add a config option for a custom url?

[vsoch]

Oh we could definitely handle that! Perhaps we should have a registry options with the url format string there?

[vsoch]

But perhaps it could be derived from the source_url instead? Can you show me your source URL?

[phue]

But perhaps it could be derived from the source_url instead? Can you show me your source URL?

I didn't think of that, that should work.

Essentially, the source_url for selfhosted instances is gitlab.example.com/<username>/<reponame> which gets served at <username>.pages.gitlab.example.com/<reponame> (see https://docs.gitlab.com/ee/administration/pages/#prerequisites)

The attached pull request should do the trick, while keeping the functionality for gitlab.com and github.com