I try to refresh the build image for CentOS 6 and can't do it now.

This directory (and version of CentOS) is deprecated. Please see this FAQ
concerning the CentOS release scheme:

https://wiki.centos.org/FAQ/General

Please keep in mind that 6.0, 6.1, 6.2, 6.3, 6.4 , 6.5, 6.6, 6.7, 6.8 , 6.9 and 6.10 no longer get any updates, nor
any security fix's.
The whole CentOS 6 is *dead* and *shouldn't* be used anywhere at *all*

http://mirror.centos.org/centos/6/readme

What our plans about Kamailio releases for RHEL 6 and Cent 6 dists?