Add Github workflow to build docker

[tbarsballe]

Simple workflow to build the docker image whenever requirements or docker config changes

Closes #395

[tbarsballe]

This is now working as expected and ready to go. Changes should be completely independent from the big refactor (#393), so it should be safe to merge in either order.

This should be squashed before merging, as most of the commits are just testing workflow behaviour.

The workflow builds the Dockerfile whenever a pull request that includes changes under the planet-notebook-docker/ path is submitted or updated - e.g. if requirements.txt changes, or the Dockerfile is changed. It will not run on PRs that don't touch files under this path. There's an argument to be made that we should try adding some caching to make the build run quicker, but I expect stuff under that path to be changed infrequently enough that this isn't worth the effort.

It only builds the Dockerfile, it does not publish it.