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.
But then you are adding one extra layer (which does has a cost) - also kinds of muddy things about what's happening. Do you think this is rebuilt often enough to be worth it?