etherpad: update to version 1.8.4

Includes reverse proxy fixes.

etherpad/Dockerfile

@@ -1,4 +1,4 @@
-FROM etherpad/etherpad:1.8.3
+FROM etherpad/etherpad:1.8.4
 
 ADD ./rootfs/defaults/settings.json /opt/etherpad-lite/settings.json