This container can be pulled via:
  docker pull registry.opensuse.org/home/dirkmueller/bci/cr/sle-15-sp6/containerfile/suse/alertmanager:0.26.0
  docker pull registry.opensuse.org/home/dirkmueller/bci/cr/sle-15-sp6/containerfile/suse/alertmanager:0.26.0-1.138

Set DOCKER_CONTENT_TRUST=1 to enable image tag verification.