Reverting to older mechanisn by re-adding Dockerfile.rocky9.base and
Dockerfile.rocky9.sdp files, but keeping the Dockerfile.rocky9 file
in place as the path forward. The intent is to see if the old
mechanism still works.
Modified run_docker_tests.sh so it'll use the old 2-image mechanism
if the *{os}.base and *.{os}.sdp files are found. Otherwise, if
the {os} file is found, it'll use that. So both old and new styles
are supported until fully deprecate one for the other.