docker: move the docker image creation inside the main repository

This is a super simple start.
At minimum, configuration via environment variables (see #3543) needs to be
integrated in Etherpad to make this user-friendly.

Resolves #3524.

docker/supervisor.conf

@@ -0,0 +1,20 @@
+[supervisord]
+nodaemon=true
+
+[unix_http_server]
+file=/var/run//supervisor.sock
+chmod=0700
+
+[rpcinterface:supervisor]
+supervisor.rpcinterface_factory = supervisor.rpcinterface:make_main_rpcinterface
+
+[supervisorctl]
+serverurl=unix:///var/run//supervisor.sock
+
+[program:etherpad]
+directory=/opt/etherpad-lite
+command=node node_modules/ep_etherpad-lite/node/server.js
+user=root
+autostart=true
+autorestart=true
+