README.md: Document how to do a local build

README.md

@@ -6,8 +6,22 @@ The BuildStream project provides
 that contain the BuildStream build tool.
 
 This repo contains the build instructions and automation that produces the
-official BuildStream Docker images. See AUTOMATION.md for more information
-on how the images are built.
+official BuildStream Docker images.
+
+How to build the images
+-----------------------
+
+The images are built by the GitLab CI pipeline for this repository, and pushed
+automatically to the Docker Hub from there. See `.gitlab-ci.yml` for how this
+is implemented.
+
+You can also build one of the images locally for testing purposes. Here is an
+example of how to do that for the 'fedora' image. These commands must be run
+from the toplevel directory of the repository.
+
+    git clone https://gitlab.com/buildstream/buildstream.git
+    mv buildstream fedora/
+    docker build fedora --tag "buildstream-fedora:my-test-version"
 
 Running BuildStream inside Docker
 ---------------------------------