fix: make: pre-mkdir the target build directory

Makefile

@@ -10,6 +10,7 @@ export
 # A container is built based on a given Dockerfile.
 # We mark a sentinel when a container is built.
 ${docker_src}/built/%.sentinel: ${docker_src}/%.dockerfile
+	mkdir -p $(dir $@);
 	grep -e '^FROM' $< | awk '{print $$2}' \
 		| tr : _ | xargs -I{} $(MAKE) ${docker_src}/built/{}.sentinel;
 	cd ${docker_src}; docker build -f $< --tag $* .;