Mac: a `make check` that checks for useless images

Signed-off-by: Akim Demaille <akim.demaille@docker.com>

docker-for-mac/Makefile

@@ -0,0 +1,10 @@
+.PHONY:
+check:
+	for img in $$(git ls-files images); do	\
+	  base=$$(basename "$$img");		\
+	  if ! git grep -q -F "$$base"; then	\
+	    echo >&2 "unused image: $$img";	\
+	    pass=false;				\
+	  fi;					\
+	done;					\
+	$$pass