ubuntu-cpc: Restore UseDomains=true

Also see https://bugs.launchpad.net/cloud-images/+bug/2106729.

Since Oracular[1]:

    Ubuntu’s systemd-networkd no longer sets UseDomains=true for managed
    network interfaces. In effect, this means that search domains
    configured in DHCP leases will not be reflected in /etc/resolv.conf
    by default. This change aligns Ubuntu’s default behavior with that
    of upstream. System administrators may choose to override this
    default on a global, or per-interface basis. See systemd.network 4
    for details.

The default in systemd is UseDomains=false. From systemd.network(5)[2]:

    DHCP=

        Furthermore, note that by default the domain name specified
        through DHCP is not used for name resolution. See option
        UseDomains= below.

    UseDomains=

        It is recommended to enable this option only on trusted
        networks, as setting this affects resolution of all hostnames,
        in particular of single-label names. It is generally safer to
        use the supplied domain only as routing domain, rather than as
        search domain, in order to not have it affect local resolution
        of single-label names.

It has been reported to us by few clouds that this breaks local name
resolution. For instance, in Google Cloud Compute, users can no longer
reach instances in the same zone[3] nor Google Cloud services[4] by
their names.

Arguably, the security concerns for having this option disabled are not
valid in cloud environments. As one of our partners said:

    IIUC, the motivation to disable UseDomains by default is that a
    laptop might be used on an untrusted network where the domains
    provided by DHCP can be a security issue, directing users to places
    they don't intend.

    But it's not possible for a cloud instance to be connected to an
    untrusted network (barring a breached account).

    The way I'm looking at this is that DHCP option 119 exists for the
    express purpose of allowing a network administrator to configure the
    DNS search path for computers on that network. I understand there's
    a security concern if that network isn't a datacenter. But in the
    cloud there's no concern (in some clouds, it's not even possible for
    DHCP response packets to come from anywhere but the cloud's own
    DHCP).

We should restore this setting in cloud images.

[1] https://discourse.ubuntu.com/t/oracular-oriole-release-notes/44878
[2] https://manpages.ubuntu.com/manpages/plucky/en/man5/systemd.network.5.html
[3] https://cloud.google.com/compute/docs/internal-dns
[4] https://cloud.google.com/compute/docs/metadata/overview

live-build/ubuntu-cpc/hooks.d/chroot/060-use-domains.chroot

@@ -0,0 +1,9 @@
+#!/bin/bash
+ 
+# See https://bugs.launchpad.net/cloud-images/+bug/2106729
+
+mkdir -p /etc/systemd/networkd.conf.d/
+cat >/etc/systemd/networkd.conf.d/50-cloudimg-settings.conf <<EOF
+[Network]
+UseDomains=true
+EOF