This container can be pulled via:
  docker pull registry.opensuse.org/devel/bci/sle-15-sp7/containerfile/containers/postfix:3.8
  docker pull registry.opensuse.org/devel/bci/sle-15-sp7/containerfile/containers/postfix:3.8-57.1

Set DOCKER_CONTENT_TRUST=1 to enable image tag verification.