Rebuild image only if environment.yml changes

[emarsden]

Binderhub seems to be building a new image for each commit to the git repository. Given that image rebuilds are very expensive, and that notebooks are likely to be updated far more often than environment.yml / requirements.txt, it seems that it would be more efficient to rebuild an image only when there is a change to a hash of whatever elements repo2docker used to generate the image.

[betatim]

This behaviour is unlikely to change. The reason for this is that (for example) what happens if you install software according to a requirements.txt can depend on any other file in the repository. We are not aware of any good way of detecting the full chain of dependencies in an automatic way.

There are attempts to speed up builds in these issues: jupyterhub/repo2docker#461 jupyterhub/repo2docker#478 and jupyterhub/repo2docker#410.

Closing this to keep the discussion focussed in the issues/PRs where there is a concrete suggestion on how to improve this. Please join in there and help test things or give feedback on the ideas.