L2SS-205: Update docker image documentation

docker-compose/README.md

@@ -25,21 +25,19 @@ https://gitlab.com/gitlab-org/gitlab-foss/-/issues/17861
 - Base images
 - Services
 
-Devices, are currently bound to the host upon building due to the intrinsics of
-how environment variables are used. They will have to be rebuild and can not be
-pulled from a local registry.
+Devices, these are detected by changes to the .yml file or directory of the
+respective service inside the docker-compose directory.
 
 Simulators, Since the source code for simulators is maintained by other teams
 we can not accurately determine from our repository if the simulator has
-changed. Instead, the images should be build by the teams their respective CI
-pipelines. We would simply pull these images as base images.
+changed. Instead, the images is build by the teams their respective CI
+pipelines. We simply pull these images as base images.
 
 Base images, these are detected by changes to the .env file in the
 docker-compose directory. When changed they will be downloaded from the remote
 registry and uploaded to our own using matching tags.
 
-Services, these are detected by changes to the .yml file or directory of the
-respective service inside the docker-compose directory.
+Services, same mechanism as devices.
 
 ### Setup and maintenance
 
@@ -50,7 +48,22 @@ docker-compose directory corresponds to one image.
 
 ### Gitlab CI phases
 
-Docker images are build and pushed in two stages. First external images are
-downloaded, retagged and pushed to our own registry.
+Docker images are managed in three phases. First is remote image storing, second
+is image building and change detection with finally image pulling.
+
+Remote images are downloaded and stored on the local registry when the .env
+file for docker-compose has changes.
+
+Local images are build when either the files in the base context directory
+change or if the docker compose file itself has changes. See the gitlab-ci.yml
+for how these changes are detected. All local images will be rebuild and tagged
+latest when a tagged commit is pushed to master.
+
+Local images download the latest image from the registry as cache unless it is
+a tagged commit on master.
 
-`git diff --name-only HEAD~1..HEAD ./`
+Finally, the integration test downloads all images from the registry either
+tagged with the current pipeline or with latest. Should both tags be unavailable
+than the integration test fails. Not all images are needed for the integration
+test. See sbin/tag_and_push_docker_image.sh for how these images are
+differentiated.