Add BUILD_DATE to buildjob

This patch allows images to leverage the a BUILD_DATE variable as part
of their build process, allowing proper continer label support.

gitlab-ci-template.yml

@@ -39,6 +39,7 @@ container-tagging:
   image: quay.io/sheogorath/build-ah-engine:1.2.0
   before_script:
     - podman login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+    - export BUILD_DATE="$(date --rfc-3339 ns)"
   script:
     - podman pull "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG-$CI_COMMIT_SHORT_SHA"
     - si-tagging -l "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG-$CI_COMMIT_SHORT_SHA" "$CI_REGISTRY_IMAGE" "${CI_REGISTRY_IMAGE_VERSION}"