Five fixes for rootless Docker setup on Arch Linux runners:
1. Write daemon.json (with containerd-snapshotter: false) BEFORE running
dockerd-rootless-setuptool.sh, so Docker 29 starts with overlay2 from
the very first boot instead of containerd snapshots.
2. Add DBUS_SESSION_BUS_ADDRESS to all systemctl --user tasks. Without it,
systemctl --user fails with "Connection refused" on Arch Linux where
the D-Bus session bus isn't auto-discovered.
3. Fix pre-configure override port driver: use 'builtin' for slirp4netns
and 'implicit' only for pasta. The previous unconditional 'implicit'
caused dockerd to exit immediately on slirp4netns hosts.
4. Fix prune timer schedule from '*-* * */6:00:00' (invalid) to
'*-*-* 00/6:00:00' (every 6 hours, valid systemd calendar spec).
5. Add explicit Docker 29→28 downgrade task for Debian/Ubuntu using
version-pinned apt install with allow_downgrades.
Closes GRM-158
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
