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.
I don't think this is necessary. Just move up the `COPY . /docker-registry` from below up here and that should fly :).