Merge pull request #1911 from keszybz/rhel-ubi

Add support for RHEL UBI

diff --cc mkosi/distributions/arch.py
Simple merge

diff --cc mkosi/distributions/centos.py
Simple merge

diff --cc mkosi/distributions/debian.py
Simple merge

diff --cc mkosi/distributions/fedora.py
Simple merge

diff --cc mkosi/distributions/opensuse.py
Simple merge

diff --cc mkosi/resources/mkosi.md
Simple merge