By enabling ‘services.openssh.startWhenNeeded’, sshd is started
on-demand by systemd using socket activation. This is particularly
useful if you have a zillion containers and don't want to have sshd
running permanently. Note that socket activation is not noticeable
slower, contrary to what the manpage for ‘sshd -i’ says, so we might
want to make this the default one day.
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.
