Develop system to update docker image

[mrocklin]

This repository tends to fall behind whenever we update Dask. We might want to move the basic image to something in a standard dask/ namespace. We may also want to add instructions on how to rebuild and republish this image whenever we issue a release. This might go into the Dask release-procedure notes

[martindurant]

Agree on adding to the notes (although the usage of the Dockerfile is mentioned right in the dask-kubernetes README). It only requires a click on docker-hub to rebuild.
Note that there are a few ideas around for building a dependency stack of pydata-related docker images.