Because of CentOS 6 End of life mirrorlist is not accessible from centos image, I've created new image based on centos:8
with changed baseurl to http://vault.centos.org, to fix this issue.
Thank for such fix information received from Error: Failed to download metadata for repo ‘appstream’: Cannot prepare internal mirrorlist: No URLs in mirrorlist — Centos 8 Fix
DockerHub Image should be available here: scorpio2002/centos-8-vault-image:8
So you can change your image from centos:8 to this one and use it.