Dockerfile for Documentcloud

This PR contains the first steps at dockerizing DocumentCloud for easier setup.

 - [x] Runs DocumentCloud via Thin behind a non-SSL nginx on Ubuntu 14.04
 - [x] PostgreSQL runs in a different container and is configured via env variables
 - [x] FileSystemStore can be used by setting an env variable
 - [ ] Solr integration works
 - [ ] Background Queue integration works
 - [ ] AWS integration works
 - [ ] Ready for production use

Documentation how to run this yourself can be found in `README.docker`. Some patches to the code base became necessary to make a non-SSL setup work etc.

We (@nickstenning and myself) will continue working on this and just wanted to start a conversation, in case some similar efforts are already ongoing.