compose: add ability to override image versions

Useful for local development, overriding to latest is possible now
without changing the compose file itself.

etherpad.yml

@@ -4,7 +4,7 @@ services:
     # Etherpad: real-time collaborative document editing
     etherpad:
         image: etherpad/etherpad:1.8.6
-        restart: ${RESTART_POLICY}
+        restart: ${RESTART_POLICY:-unless-stopped}
         environment:
             - TITLE=${ETHERPAD_TITLE}
             - DEFAULT_PAD_TEXT=${ETHERPAD_DEFAULT_PAD_TEXT}