gitlab-ci: Accept $BUILD_ID CI var to create additional tags

When triggering a pipeline, by setting the `BUILD_ID` CI variable it is
now possible to tell the pipeline to create `$IMAGE:$BUILD_ID` tags
like `v2022dev0-image-builder:20201225.000`.

With this it is possible to have an external orchestrator trigger all
the nightly pipelines with a shared build id, rather than have each
pipeline compute one at different times.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -84,6 +84,13 @@ build-base-rootfs:
     - echo "Tagging ${TEST_TAG} as ${TAG} for ${IMAGE}"
     - skopeo copy --src-creds "$AUTH" --dest-creds "$AUTH" docker://"${IMAGE}:${TEST_TAG}" docker://"${IMAGE}:${TAG}"
     - echo "🏷️ Tagged ${IMAGE}:${TAG}"
+    - |
+      if [ -n "${BUILD_ID:-}" ]
+      then
+        echo "Tagging ${TEST_TAG} as ${BUILD_ID} for ${IMAGE}"
+        skopeo copy --src-creds "$AUTH" --dest-creds "$AUTH" docker://"${IMAGE}:${TEST_TAG}" docker://"${IMAGE}:${BUILD_ID}"
+        echo "🏷️ Tagged ${IMAGE}:${BUILD_ID}"
+      fi
 
 build-base-docker-image:
   extends: .build-docker-image