Could not resolve host: mirrorlist.centos.org; Unknown error

centos7のdocker imageを使用して環境構築時に、yumコマンド実行で突如以下のようなエラーが発生するようになった(20240701~)
```
0.532 Loaded plugins: fastestmirror, ovl
0.710 Determining fastest mirrors
6.226 Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=container error was
6.226 14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
6.230 
6.230 
6.230  One of the configured repositories failed (Unknown),
6.230  and yum doesn't have enough cached data to continue. At this point the only
6.230  safe thing yum can do is fail. There are a few ways to work "fix" this:
6.230 
6.230      1. Contact the upstream for the repository and get them to fix the problem.
6.230 
6.230      2. Reconfigure the baseurl/etc. for the repository, to point to a working
6.230         upstream. This is most often useful if you are using a newer
6.230         distribution release than is supported by the repository (and the
6.230         packages for the previous distribution release still work).
6.230 
6.230      3. Run the command with the repository temporarily disabled
6.230             yum --disablerepo=<repoid> ...
6.230 
6.230      4. Disable the repository permanently, so yum won't use it by default. Yum
6.230         will then just ignore the repository until you permanently enable it
6.230         again or use --enablerepo for temporary usage:
6.230 
6.230             yum-config-manager --disable <repoid>
6.230         or
6.230             subscription-manager repos --disable=<repoid>
6.230 
6.230      5. Configure the failing repository to be skipped, if it is unavailable.
6.230         Note that yum will try to contact the repo. when it runs most commands,
6.230         so will have to try and fail each time (and thus. yum will be be much
6.230         slower). If it is a very temporary problem though, this is often a nice
6.230         compromise:
6.230 
6.230             yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true
6.230 
6.230 Cannot find a valid baseurl for repo: base/7/x86_64
```

### Dockerfile
```dockerfile
FROM centos:7

RUN yum -y update && yum clean all && \
    yum -y install ...
```

## 原因
centos7のサポート終了により、`mirrorlist.centos.org`が存在しなくなった様子
>CentOS Linux 7 は、2024 年 6 月 30 日にサポート終了 (EOL) となります

https://www.redhat.com/ja/topics/linux/centos-linux-eol

## 解決策
`/etc/yum.repos.d/CentOS-*.repo`からmirrorlistのコメントアウトとbaseurlの書き換えを行い、サポート終了後のリポジトリを参照するように

e.g. /etc/yum.repos.d/CentOS-Base.repo
* before
```
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
...
```
* after
```
[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
baseurl=http://vault.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
...
```
dockerfileでこの書き換えをyum実行前に指定する
```dockerfile
FROM centos:7

RUN sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-* && \
    sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-* && \
    yum -y update && yum clean all && \
    yum -y install ...
```

ref : https://stackoverflow.com/questions/78692851/could-not-retrieve-mirrorlist-http-mirrorlist-centos-org-release-7arch-x86-6