Instructions for running in a docker container.

If you have docker installed, just run `docker build .` in the directory
containing `Dockerfile` and you will get an image that runs
hackage-server with the default settings in an isolated environment.

A prebuilt image is available at
https://index.docker.io/u/zsol/hackage-server/, which you can use by
running `docker run zsol/hackage-server`.