Update dockerfiles to use volumes to manage source

[bcongdon]

We can improve the dev experience on Groot by using Docker volumes instead of using ADD to include source in the docker image.

[bcongdon]

Doing some more digging on this, I don't think we should change Dockerfiles. The idiomatic way of creating Dockerfiles is to bake the source in with the container image.

However, we can add a 'development' docker command to the docs, so that you can mount a service's source directory inside the container for development.

Something like:
docker run -v `pwd`:/usr/src/app <container_id>