cleanup dangling images on make rm

Change-Id: Iab64acda90bf5de29732ff6c1b4bdc4361370f42
diff --git a/test-standalone/Makefile b/test-standalone/Makefile
index ffbe368..ee3a206 100644
--- a/test-standalone/Makefile
+++ b/test-standalone/Makefile
@@ -139,6 +139,7 @@
 rm: stop
 	test ! -s $(DOCKER_COMPOSE_YML) || sudo docker-compose -p $(DOCKER_PROJECT) -f $(DOCKER_COMPOSE_YML) rm -f
 	sudo docker-compose -p $(BOOTSTRAP_PROJECT) -f $(BOOTSTRAP_YML) rm -f
+	[ -n "$(shell docker images -f \"dangling=true\" -q)" ] && docker rmi $(shell docker images -f "dangling=true" -q) || true
 
 docker-clean:
 	sudo docker rm -f $(shell sudo docker ps -aq)