etherpad: pin image version

Makefile

@@ -34,7 +34,6 @@ clean:
 
 prepare:
 	docker pull debian:stretch-slim
-	docker pull etherpad/etherpad
 	FORCE_REBUILD=1 $(MAKE)
 
 .PHONY: all build tag push clean prepare

etherpad/Dockerfile

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