Add Dockerfile and config reading db/url from env

This includes a config which will read env variables and expect a linked mysql container. It would be run like this:

    docker run -link mysql-server:db -e DB_NAME=ghost -e DB_USER=ghost -e DB_PASSWORD=foobar23 ghost

I know that not everyone is using docker, but if you search for 'docker ghost blog' there are a lot people running ghost on docker and I bet they would be happy to have a always up to date Dockerfile right in the source. Beside that, you could add this repo to index.docker.io as trusted build and installing a always up to date ghost would be just a 'docker pull <user>/ghost'.

So what do you think?