Publish the (repo, commit, repo2docker version) in build cache?

[arnim]

Even after adding a handler to expose what versions a hub uses it is still not certain that what launches at mybinder.org successfully does so as well at a different hub. Furthermore, smaller deployments may only have a limited number of repos already in the cache. To support a consistent UX across deployments, publishing which (repo, commit, repo2docker version) triples are in the build cache of a deployment may be of use in the future.

Suggested in
#821 (comment)

[arnim]

The issue is also discussed in #998