Dockerfile

This adds a Dockerfile and docker use instructions to the readme. 

However, it is not the ideal Docker setup. Ideally, we would want a repo like [this from discourse](https://github.com/discourse/discourse_docker) that provides a script for managing containers and handles persistence of the database. This would allow us to provide a simple install guide like [such](https://github.com/discourse/discourse/blob/master/docs/INSTALL-digital-ocean.md) and still expose lower level commands. 

Anyways, questions, comments, concerns welcome. 
thanks for this - useful!