Add Docker configuration for deployment

This PR gives us the ability to build a Docker container of the app which is suitable for deployment. It follows most of the [12-factor app conventions](http://12factor.net/). Some of this is bound to evolve over time, but this works pretty well now.

To build and test locally follow these steps:
  1. Start up your Docker environment
  2. Build the container: `$ rake deploy:build`
  3. To test the build locally: `$ rake deploy:qa`

--> The container should be available at `http://$DOCKER_HOST`