Edit: /usr/lib/tmpfiles.d/systemd-nspawn.conf (976B)
# This file is part of systemd.
#
# systemd is free software; you can redistribute it and/or modify it
# under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation; either version 2.1 of the License, or
# (at your option) any later version.
# See tmpfiles.d(5) for details
Q /var/lib/machines 0700 - - -
# Remove old temporary snapshots, but only at boot. Ideally we'd have
# "self-destroying" btrfs snapshots that go away if the last
# reference to it does. To mimic a scheme like this at least remove
# the old snapshots on fresh boots, where we know they cannot be
# referenced anymore. Note that we actually remove all temporary files
# in /var/lib/machines/ at boot, which should be safe since the
# directory has defined semantics. In the root directory (where
# systemd-nspawn --ephemeral places snapshots) we are more strict, to
# avoid removing unrelated temporary files.
R! /var/lib/machines/.#*
R! /.#machine.*