build: add prepare command

Makefile

@@ -32,4 +32,9 @@ clean:
 	docker-compose rm
 	docker network prune
 
-.PHONY: all build tag push clean
+prepare:
+	docker pull debian:stretch-slim
+	docker pull etherpad/etherpad
+	FORCE_REBUILD=1 $(MAKE)
+
+.PHONY: all build tag push clean prepare