This container can be pulled via:
  docker pull registry.opensuse.org/filesystems/ceph/images/opensuse/prometheus/prometheus-alertmanager:0.26.0
  docker pull registry.opensuse.org/filesystems/ceph/images/opensuse/prometheus/prometheus-alertmanager:0.26.0.3.1815
  docker pull registry.opensuse.org/filesystems/ceph/images/opensuse/prometheus/prometheus-alertmanager:latest
  docker pull registry.opensuse.org/filesystems/ceph/images/opensuse/prometheus/prometheus-alertmanager:tumbleweed

Set DOCKER_CONTENT_TRUST=1 to enable image tag verification.