[Bug 2165422] Re: setup_mountpoint required before installing packages for 26.04 buildd image

[Launchpad Bug Tracker]

** Merge proposal linked:
   https://code.launchpad.net/~allenab/livecd-rootfs/+git/livecd-rootfs/+merge/511837

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Title:
  setup_mountpoint required before installing packages for 26.04 buildd
  image

Status in livecd-rootfs package in Ubuntu:
  Invalid
Status in livecd-rootfs source package in Resolute:
  New

Bug description:
  [Impact]
  ubuntu-base buildd image builds have been failing on Resolute since 2026-08-05 (and on Stonking). On Resolute the build aborts in config/hooks/03-network.binary with:

  ```
  Selecting previously unselected package udev.
  Preparing to unpack .../udev_259.5-0ubuntu3.4_amd64.deb ...
  Unpacking udev (259.5-0ubuntu3.4) ...
  Setting up systemd (259.5-0ubuntu3.4) ...
  Cannot open '/etc/machine-id' in neither writable nor read-only mode: Function not implemented
  dpkg: error processing package systemd (--configure):
   old systemd package postinst maintainer script subprocess failed with exit status 1
  dpkg: dependency problems prevent configuration of udev:
   udev depends on systemd | systemd-standalone-sysusers | systemd-sysusers; however:
    Package systemd is not configured yet.
    Package systemd-standalone-sysusers is not installed.
    Package systemd-sysusers is not installed.
    Package systemd which provides systemd-sysusers is not configured yet.

  dpkg: error processing package udev (--configure):
   dependency problems - leaving unconfigured
  Setting up libkmod2:amd64 (34.2-2ubuntu2) ...
  dpkg: dependency problems prevent configuration of systemd-resolved:
   systemd-resolved depends on systemd (= 259.5-0ubuntu3.4); however:
    Package systemd is not configured yet.
  ```

  
  Both are the same root cause: 03-network.binary runs in live-build's binary stage, after umount chroot/proc, and installs a package whose maintainer script requires proc.
  The Stonking fix was raised at https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/511182 and is already landed. This MP is a cherry-pick of commit to ubuntu/resolute, with no changes beyond the changelog entry.

  [Test Plan]
  Reproduce:
  With livecd-rootfs from resolute-updates installed on the builder, trigger a cpc-buildd / ubuntu-base livefs build for Resolute amd64.
  Observe the build fail in config/hooks/03-network.binary with the Cannot open '/etc/machine-id' ... Function not implemented error above (reference failing build 1026983, log lines 38182–38222).
  Verify the fix:
  Install livecd-rootfs from resolute-proposed and re-trigger the same build for amd64 and at least one other architecture (e.g. arm64).
  Confirm the build completes

  [Where problems could occur]
  The deleted hook also ran apt-get update and apt-get clean in the chroot during the binary stage. Any later buildd binary hook that implicitly relied on freshly refreshed apt lists would now need its own apt-get update. The remaining buildd hooks were reviewed and each does its own update.
  52-linux-virtual-image.binary no longer installs udev explicitly and now depends on auto/config seeding it. If that seeding is removed in a future change, the virtual image would silently lose udev.
  The change is confined to the buildd subproject; no other flavour, project or image type shares these code paths, so the blast radius is limited to buildd/builder images.

  [Other Info]
  Fix commit: 5d43ef14befe061861d6ed67946a20891279fc90, cherry-picked unchanged from the development series (Stonking).

  LP: #2165421, MP #511182

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