add docker support

I have added support in the documentation for how to run `a11y` in a docker container. I have also added a `Dockerfile`, so that you can set up Automated Docker builds, for the repository.

For it to create a docker build of the server, on every git push, you would need to set up an account on the docker registry and [add this repo to build](https://registry.hub.docker.com/builds/github/addyosmani/a11y) `addyosmani/a11yr`. 

It should pull from the master branch.

How does that sound?