ClefOS is official

Or more precisely, ClefOS is an official image. Official image means, it is part of curated content on Docker Hub. It is also (AFAIK) the first official image that has no support yet for other platforms. Well, this might be a dubious comparison since it is actually an equivalent to CentOS which does not have any s390x backing. ClefOS can't be named CentOS, since Neale Ferguson and company at Sine Nomine Associates are not part of the CentOS organization. However, as you can see it is very closely following it:

$ docker run -ti clefos
Unable to find image 'clefos:latest' locally
latest: Pulling from library/clefos
26dbd8e1d5ff: Pull complete
Digest: sha256:8e89216b23e7a5716a7e31de352ed777769738d258a1e20cc9cff06e39316717
Status: Downloaded newer image for clefos:latest
bash-4.2# cat /etc/os-release
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

bash-4.2#

This gives you a great option for enabling CentOS (or RHEL) based images on IBM Z. The identification above is technically very sane, since some projects check the distribution, and for some reason do not know ClefOS, but know CentOS.