When executing
> openstack overcloud container image prepare --namespace docker-registry.engineering.redhat.com/rhosp13 \
> --tag 2018-01-12.2 --prefix openstack- \
> --set ceph_namespace=registry.access.redhat.com/rhceph \
> --set ceph_image=rhceph-3-rhel7 --set ceph_tag=3-2 \
> --output-images-file /home/stack/container_images.yaml \
> --environment-file /usr/share/openstack-tripleo-heat-templates/environments/services/sahara.yaml \
> --environment-file /usr/share/openstack-tripleo-heat-templates/environments/cinder-backup.yaml

It generates empty list of images:
> container_images: []

Same command when used before for Pike/12 worked.

To generate non empty list, both sahara.yaml and cinder-backup.yaml had to be removed, just one of them still results in empty list.
List obtained after removing both can be see e.g. in other bugs attachment https://bugzilla.redhat.com/attachment.cgi?id=1382579 .

cat /usr/share/openstack-tripleo-heat-templates/environments/services/sahara.yaml
> resource_registry:
>   OS::TripleO::Services::SaharaApi: ../../puppet/services/sahara-api.yaml
>   OS::TripleO::Services::SaharaEngine: ../../puppet/services/sahara-engine.yaml

cat /usr/share/openstack-tripleo-heat-templates/environments/cinder-backup.yaml
> resource_registry:
>   OS::TripleO::Services::CinderBackup: ../puppet/services/pacemaker/cinder-backup.yaml
>   # For non-pcmk managed implementation
>   # OS::TripleO::Services::CinderBackup: ../puppet/services/cinder-backup.yaml(undercloud)