Switch to "FROM python:2.7"

Turns out, it "just works" as-is. :)

(Once I got past #518, of course.)

As requested in https://github.com/docker/docker-registry/pull/620#issuecomment-58964086.
That directory changes very rarely, while the rest changes very often.

Now, the dockerfile is not used at all during development (the workflow is simply not practical): `python setup.py develop` + gunicorn running in debug mode does pick-up changes without the need to rebuild anything (not to mention the extra VM slowness if working on OSX). Even tests are run much faster using plain virtualenvs, also giving much more flexibility (can test multiple different versions of python).

Color me heretic :)