Switch order of add and pip install

This just updates the Dockerfile to take advantage of the ADD caching
added in 0.7.3 to speed up docker builds of the registry.

Docker-DCO-1.1-Signed-off-by: Phil Kates <me@philkates.com> (github: philk)
Merge pull request #257 from philk/use_add_caching

Switch order of add and pip install