The ability for unprivileged users to mount external media is useful
regardless of the desktop environment. Also, since udisks2 is
activated on-demand, it doesn't add any overhead if you're not using it.
This used to work with systemd-nspawn 203, because it bind-mounted
/etc/resolv.conf (so openresolv couldn't overwrite it). Now it's just
copied, so we need some special handling.