Revert "drm_hwcomposer: CI: Use upstream container image conditionally"

This reverts commit 7aff49bb901652a8f1cc806b06de8e7f86bfc6b6.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 5863159..ea15c9a 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -5,7 +5,7 @@
 
 variables:
   DEBIAN_FRONTEND: noninteractive
-  DOCKER_IMAGE_TAG: registry.freedesktop.org/drm-hwcomposer/drm-hwcomposer:latest
+  DOCKER_IMAGE_TAG: $CI_REGISTRY_IMAGE:latest
 
 stages:
   - build-container
@@ -20,19 +20,13 @@
     - 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: