This updates our etherpad image to version 1.18.16. To do this we've had
to adopt the upstream Dockerfile locally and introduce our own edits as
upstream hasn't build images for this release or the previous one.
Minor updates are made to the upstream Dockerfile to update the
maintainer directive and convert from using a local copy of the source
code to a git clone.
Change-Id: I561680072085caff751e08b6f2fd79dee1d4efe8
ce67fad1f5