Add official image on Dockerhub with instructions for easy use

[geerlingguy]

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.

[BeardMatt]

Thumbs up for this!!! :)

[ShadwDrgn]

same