Add support for pinning repo2docker version

One thing missing for fully reproducible environments is a mechanism for pinning the version of repo2docker.

Several people have proposed ideas and it seems we have a plan: repo2docker will start, inspect a /binder/repo2docker.version file that specifies the version (git commit or version) that the user wants to use and then restart/start a new copy of repo2docker in that version.

The way I’d start on this is to experiment with adding a new entrypoint/CLI script that starts the current repo2docker CLI script inside a container together with the right mounts(?). Docker hub has a collection of images for lots of git revisions which we can use.

(related #170)

[betatim]

IMHO running from the CLI should do the same thing as using repo2docker as a library. This means that running repo2docker https://github.com/org/repo does the following:

1. fetch the repository
2. inspect the repository to find the version of repo2docker it specifies; default to the version of repo2docker that is installed
3. run docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock -e DOCKER_HOST=unix:///var/run/docker.sock jupyter/repo2docker:<r2dversionhere> ... with the ... replaced by the original CLI arguments

This is the MVP I’d try to implement and see what happens/breaks. One thing that could get tricky is to get repo2docker /path/to/local/directory to work properly when forwarding the CLI arguments.

I think almost all uses of repo2docker right now are via the CLI, I don’t know many who use repo2docker as a library. So I think we should aim for pip install jupyter-repo2docker; repo2docker https://gitlab.com/org/repo-with-version-specified-in-it to “just work” as something you can type into a terminal.

[manics]

At some point we’ll have to update the R2D base image, so I guess this will become more critical.