This improves Docker support in the following ways:

* Base image off `ruby:onbuild` to avoid unnecessary custom steps and ease
derived images.
* Adds `.dockerignore` to avoid sending large files to build.
* Adds a `docker-compose.yml` so users can simply type `docker-compose up`
to get it running.
* Explicity sets the volume definition in the `Dockerfile`.
* Volume is automatically mount by `docker-compose` (the `source` directory
relative to the project's dir).
* Workaround watcher failing to pick directory changes probably because of
vboxsf and the `listen` gem failing to register filesystem changes.
* Updates documentation.