$ docker run --rm ppc64le/centos:7 /bin/bash -c "cat /etc/redhat-release && yum makecache && yum-builddep --assumeno bash"
CentOS Linux release 7.7.1908 (AltArch)
Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
 * base: mirror.vcu.edu
 * extras: mirror.vcu.edu
 * updates: ftp.osuosl.org
Metadata Cache Created
Loaded plugins: fastestmirror, ovl
Enabling base-source repository
Enabling extras-source repository
Enabling updates-source repository
Loading mirror speeds from cached hostfile
 * base: mirror.vcu.edu
 * extras: mirror.vcu.edu
 * updates: ftp.osuosl.org
http://vault.centos.org/altarch/7/extras/Source/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below wiki article 

https://wiki.centos.org/yum-errors

If above article doesn't help to resolve this issue please use https://bugs.centos.org/.

failure: repodata/repomd.xml from extras-source: [Errno 256] No more mirrors to try.
http://vault.centos.org/altarch/7/extras/Source/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found