Docker hub pages have outdated instructions

[Clayton Whitelaw]

https://hub.docker.com/r/sphinxdoc/sphinx

https://hub.docker.com/r/sphinxdoc/sphinx-latexpdf

These pages instruct the user to run `make` which doesn't seem to work, and the commands are different on the github README so I assume the docker page is just old.

And even on the github repo, I don't think stuff like "sphinx-build -M latexpdf . _build" is required because the Dockerfile CMD lines seem to take care of that.