Add official image on Dockerhub with instructions for easy use

If you want to just run the container straightaway and not have to build it or run any Node.js on your system, I could have an official image hosted on Docker Hub. When you run it you can pass in your own path to a config.json file (e.g. in the same directory as a `docker-compose.yml` file that you copy from the README), and it will pick up that configuration.

Inspired by #3. 