I traced it down to:
http://pkgs.fedoraproject.org/cgit/rpms/atomic.git/commit/?h=f25&id=7...
http://pkgs.fedoraproject.org/cgit/rpms/atomic.git/commit/?h=f25&id=c...
I.e. atomic needs to build RPMs for the system container
work.
Some of the new packages are direct requires of rpm-build,
while the others are due to rpm-build requiring
/usr/bin/gdb-add-index, while pulls in gdb-headless, which
recommends the dnf stuff as Colin mentioned.
Related:
https://github.com/projectatomic/rpm-ostree/issues/731
Hmm, wonder how feasible it'd be to build these RPMs in a
runc container instead?