Fix doc Dockerfile
Summary:
The output structure of the documentation generator sphinx changed.
The html output is now in _build/ and no longer in _build/html.
This broke the gerneration of the docker image.
This diff fixes the image, and specifies the version of sphinx directly,
so this does not happen again.
Test Plan: Build the image an run it. Check via browser if the docs work.
Reviewers: WyldBot
Reviewed By: WyldBot
Differential Revision: https://phabricator.wildfyre.net/D233