https://github.com/rpm-software-management/mock/wiki/Release-Notes-1.4.1
The latest mock release now uses nspawn by default instead of chroot. There are going to be some installations / builders where chroot is still required. Koji should provide a mechanism to choose between nspawn and chroot and have that be configurable. I think it would make sense for the granularity of configuration to be at the builder level.
PR for discussion: #400
Metadata Update from @tkopecek: - Issue set to the milestone: 1.13
Commit 01277c13 fixes this issue
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/398
Please continue any further discussion there.