Backported from thewtex/docker-centos-build@26f7fd3
CentOS 5 is no longer supported and requests to obtain information from
the mirrors fails. However, the packages are still available on the
"Vault" mirrors.
Comment out the `mirrorlist` entries and set `enabled=0` in
CentOS-Base.repo, add entries for 5.11 in CentOS-Vault.repo and mark
them `enabled=1`.
