drm_hwcomposer: CI: Use upstream container image conditionally

Right now we use $CI_REGISTRY_IMAGE:latest as the docker image for
running build, tidy and checkstyle.

The problem with this is that CI_REGISTRY_IMAGE points to the *fork*'s
container registry.

For example, if the ddavenport user has this fork:
https://gitlab.freedesktop.org/ddavenport/drm-hwcomposer

Then CI_REGISTRY_IMAGE will be:
registry.freedesktop.org/ddavenport/drm-hwcomposer

This won't work for forks that never build the container image.

Use registry.freedesktop.org/drm-hwcomposer/drm-hwcomposer:latest as
container image by default.

For MRs that modify .ci/Dockerfile, a container will be rebuild
and the forks image will be used instead.

Link: https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer/-/issues/97
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

.gitlab-ci.yml

@@ -5,7 +5,7 @@ workflow:
 
 variables:
   DEBIAN_FRONTEND: noninteractive
-  DOCKER_IMAGE_TAG: $CI_REGISTRY_IMAGE:latest
+  DOCKER_IMAGE_TAG: registry.freedesktop.org/drm-hwcomposer/drm-hwcomposer:latest
 
 stages:
   - build-container
@@ -20,13 +20,19 @@ build-container:
     - docker:27.0.3-dind
   variables:
     DOCKER_TLS_CERTDIR: ""
+    # Use the fork's docker image when rebuilding the container
+    DOCKER_IMAGE_TAG: $CI_REGISTRY_IMAGE:latest
   before_script:
     - echo "$CI_JOB_TOKEN" | docker login $CI_REGISTRY -u $CI_REGISTRY_USER --password-stdin
   script:
     - docker build -t $DOCKER_IMAGE_TAG -f .ci/Dockerfile .
     - docker push $DOCKER_IMAGE_TAG
+    - echo "DOCKER_IMAGE_TAG=$DOCKER_IMAGE_TAG" >> build.env
   after_script:
     - docker logout
+  artifacts:
+    reports:
+      dotenv: build.env
   rules:
     - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
       changes: