gitlab-ci-template: fix using custom Dockerfile

When using a custom Dockerfile the podman build command also needs to
know about this with the -f flag.

gitlab-ci-template.yml

@@ -30,6 +30,7 @@ variables:
       $CI_REGISTRY_BUILD_ARGS
       -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG-$CI_COMMIT_SHORT_SHA-$CI_REGISTRY_IMAGE_ARCH"
       --format docker
+      -f "${CI_REGISTRY_BUILD_DOCKERFILE:-./Dockerfile}"
       "${CI_REGISTRY_BUILD_CONTEXT:-.}"
     - podman push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG-$CI_COMMIT_SHORT_SHA-$CI_REGISTRY_IMAGE_ARCH"
     - podman push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG-$CI_COMMIT_SHORT_SHA-$CI_REGISTRY_IMAGE_ARCH" "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG-latest-$CI_REGISTRY_IMAGE_ARCH"