Add a Dockerfile for building Drone.io Docker containers.

With the help/direction of @bradrydzewski and @crosbymichael, I have created the Dockerfile included in this commit.  I've tested this locally and was able to successfully create a Docker container image for Drone and run it using Docker.  This will be a huge help for people wanting to develop/run Drone without being on a Linux box.

I tested this on OS X 10.9.1 using boot2docker 0.5.4 and Docker 0.8.0.