Fix uid/gid for centos_x86_64:9
It seems most other Dockerfiles use {uid|gid}=5005. Also this seems to be overriden here anyway: https://gitlab.cern.ch/cernvm/build-images/-/blob/master/.gitlab-ci.yml#L24
Also not sure if the bug referenced in the Dockerfile (https://gitlab.cern.ch/cernvm/build-images/-/blob/master/docker/centos_x86_64%3A8.Dockerfile#L5) is still relevant, but I copied a comment to the centos9 one to clarify the comment below.