在centos 7環境下跑centos 6 docker image
docker run -it centos:6
下 yum update 報錯，顯示無法retrieve repomd.xml
確認一下原因，主要是因為 centos 6在2020 Nov就停止支援了，參考https://forums.centos.org/viewtopic.php?t=72710
Red Hat have pulled the plug on RHEL 6.x as of Nov 30th 2020 and as a result CentOS 6 is now a dead version. The online yum repos for CentOS 6 have been archived to vault.centos.org and there will be no more updates to it at all, ever.
yum repo archive到 vault.centos.org ，所以須將yum相關config修改，直接修改/etc/yum.repos.d/CentOS-Base.repo
將所有mirrorlist註解掉，改成baseurl指向vault.centos.org 就可以了
內容類似在/etc/yum.repos.d的CentOS-Vault.repo
不過 CentOS-Vault.repo 主要是archive packages obsoleted by current release，所以以CentOS 6.10來說 它裡面只包含到CentOS 6.9 repo
如果是內部要長期維護的話，最好還是建一個local yum repo mirror，避免網路上的archive repo失效