fix: derive buildah cache from snapshot image

templates/gitlab-ci-docker.yml

@@ -586,12 +586,13 @@ docker-buildah-build:
   extends: .docker-base
   stage: package-build
   image: "$DOCKER_BUILDAH_IMAGE"
-  variables:
-    BUILDAH_BUILD_CACHE: "$CI_REGISTRY_IMAGE/snapshot/cache"
   script:
     - configure_containers_registries
+    # derive buildah_build_cache repository
+    - buildah_build_cache="${DOCKER_SNAPSHOT_IMAGE%:*}/cache"
+    - log_info "Using ${buildah_build_cache} as build cache repository"
     # build and push image
-    - buildah build --file "$DOCKER_FILE" --tag $DOCKER_SNAPSHOT_IMAGE --layers --cache-from $BUILDAH_BUILD_CACHE --cache-to $BUILDAH_BUILD_CACHE --build-arg http_proxy="$http_proxy" --build-arg https_proxy="$https_proxy" --build-arg no_proxy="$no_proxy" $DOCKER_METADATA $DOCKER_BUILD_ARGS "$(docker_context_path)"
+    - buildah build --file "$DOCKER_FILE" --tag $DOCKER_SNAPSHOT_IMAGE --layers --cache-from $buildah_build_cache --cache-to $buildah_build_cache --build-arg http_proxy="$http_proxy" --build-arg https_proxy="$https_proxy" --build-arg no_proxy="$no_proxy" $DOCKER_METADATA $DOCKER_BUILD_ARGS "$(docker_context_path)"
     - buildah push --digestfile .img-digest.txt "$DOCKER_SNAPSHOT_IMAGE"
     # display digest of the resulting image
     - cat .img-digest.txt