Clean up how a variable gets set up

Makefile

@@ -7,7 +7,7 @@ include /devel/docker/docker_vars.sh
 export
 
 ${docker_src}/built/%.sentinel: ${docker_src}/%.dockerfile
-	grep -e '^FROM' "$*".dockerfile | awk '{print $$2}' \
+	grep -e '^FROM' $< | awk '{print $$2}' \
 		| tr : _ | xargs -I{} $(MAKE) ${docker_src}/built/{}.sentinel;
 	cd ${docker_src}; docker build -f $< --tag $* .;
 	touch $@;