C�dric Bosdonnat found that the problem is caused by a left over
gpg-agent process in the chroot after yum/dnf runs. I cannot
reproduce this myself on Fedora 22 host, but that certainly
doesn't mean it doesn't happen on SUSE host, and I would look at
a patch if one were to be posted upstream.
In the meantime, this is a plausible (untested) workaround. Replace
'--install mc' with:
--run-command '
dnf -y install mc
pkill gpg-agent
'