Fix odd issues with "make docs", add "make docs-shell", and canonicalize our docs Dockerfile a bit more

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3598 from tianon/fix-vieux-docs-errors-again

Fix odd issues with "make docs", add "make docs-shell", and canonicalize...