Create Dockerfile for doc generation

[sorrell]

It would be nice to have a Dockerfile for sphinx/docs generation for local development instead of installing all of the libs/dependencies natively on each machine.

Ideally, the Dockerfile would document the dependencies and could be built with something like: docker build -t age-docs .

Once that image is built locally, I could then make changes to the rst/md files, and rebuild the docs by mapping in my local directory into the container, like: docker run -v $(pwd):/app age-docs