We need to install the ep_headings plugin. We've got a hack in place
to do it in prod, but it seems like every time we npm install the
plugin in ansible, we need to restart the etherpad container
for $reasons.
It's cleaner to just build an image based on upstream with ep_headings
installed.
Change-Id: Id3b31bbabbbc5f6dcfe56486e48916384292f693
7657b69387