This container can be pulled via:
  docker pull registry.opensuse.org/home/kukuk/container/opensuse/postfix:3.9.1
  docker pull registry.opensuse.org/home/kukuk/container/opensuse/postfix:3.9.1-5.121
  docker pull registry.opensuse.org/home/kukuk/container/opensuse/postfix:latest

Set DOCKER_CONTENT_TRUST=1 to enable image tag verification.