Add Dockerfile for building a production-like image

The image uses Nginx and uWSGI and tries to re-use the production
settings as much as possible.
Improve Nginx regexes for testing if request is internal
Wrap private uploads with an authorisation view

This uses Nginx's support for X-ACCEL-REDIRECT headers to allow a
private files to be served.
Update comments in Dockerfile and urls.py
Merge pull request #25 from tangentlabs/private-media

Allow private media to be downloaded with authentication