dockerfile: switch to vault repo for centos 7

[crazy-max]

relates to https://github.com/tonistiigi/xx/actions/runs/10112723728/job/27985481000#step:6:423

#21 1.333 + yum update -y
#21 1.426 Loaded plugins: fastestmirror, ovl
#21 1.469 Loading mirror speeds from cached hostfile
#21 1.530 Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=container error was
#21 1.530 14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"

CentOS 7 has reached EOL on June 30: https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-stream-8-and-centos-linux-7. Switch to vault repo.