Add documentation on how to build a QA Docker image

qa/README.md

@@ -80,4 +80,14 @@ GITLAB_USERNAME=jsmith GITLAB_PASSWORD=password GITLAB_SANDBOX_NAME=jsmith-qa-sa
 
 All [supported environment variables are here](https://gitlab.com/gitlab-org/gitlab-qa#supported-environment-variables).
 
+### Building a Docker image to test
+
+Once you have made changes to the CE/EE repositories, you may want to build a
+Docker image to test locally instead of waiting for the `gitlab-ce-qa` or
+`gitlab-ee-qa` nightly builds. To do that, you can run from this directory:
+
+```sh
+docker build -t gitlab/gitlab-ce-qa:nightly .
+```
+
 [GDK]: https://gitlab.com/gitlab-org/gitlab-development-kit/