Make build image depend on tools subdir

Missed from 0164d24583

Makefile

@@ -244,7 +244,7 @@ endif
 
 # This rule makes sure the build image is up-to-date.
 # It also makes sure the multiarch hooks are reqistered in the kernel so the QEMU emulation works
-$(BUILD_UPTODATE): build/*
+$(BUILD_UPTODATE): build/* tools/.git
 	$(SUDO) docker build -t $(BUILD_IMAGE) build/
 	$(SUDO) docker tag $(BUILD_IMAGE) $(BUILD_IMAGE):$(shell tools/image-tag)
 	$(SUDO) docker run --rm --privileged multiarch/qemu-user-static:register --reset