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's up to you -- don't you have to rebuild often while doing development on it?  I'm not worried about space as much as time.  How often does this directory change vs the rest of the codebase?