fixup! fixup! README.md: add build and testing instructions

Author: crasbe

README.md

@@ -23,6 +23,9 @@ You can build the image with the following command. Setting the
 instead of the upstream version. You can omit this parameter if you haven't
 made any changes to `riotdocker-base`.
 
+***NOTE:*** If docker complains about not finding the image, you can try to
+set the `DOCKER_REGISTRY` argument to `localhost` instead.
+
 ```sh
 docker build --build-arg DOCKER_REGISTRY=docker.io/library -t static-test-tools ./static-test-tools/
 ```