Refresh installation method

Summary:
The Etherpad repository now includes a Dockerfile, with a reverse
procedure installation: first the plugins, then Etherpad though npm link.

Apply this method to ease image future maintenance.

Test Plan: Tested on pad.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3261