Currently, Image Builder allows to add extra mountpoints only to a pre-defined set of locations. However, customers demand more location. Let's make them happy.
 

Goal:

• Allow users to specify arbitrary extra mountpoints.
• Implement a denylist for mountpoints that simply cannot be backed by a physical partition:
  • /bin, /sbin, /lib, /lib64, /proc, /sys, /dev, /lost+found, /sysroot (ostree only?), /run, /boot/efi
  • Consult https://www.freedesktop.org/wiki/Software/systemd/APIFileSystems/ for more ideas

Acceptance Criteria:

• Image Builder has loosened rules for custom mountpoints.
• This is available both in the hosted service and on-prem (including UIs).

Open questions:

• systemd changes its support for separate /usr. We need to check how it affects IB. See https://github.com/systemd/systemd/blob/82b7bf8c1c8c6ded6f56b43998c803843a3b944b/NEWS#L7
  •  [thozza] This does not have effect on IB, since our policy does not support custom mountpoints under /usr and we also do not support custom mountpoints for paths which are merged into /usr. Having /usr on a separate partition is supported, since systemd fstab generator handles it in initram and mounts it before switching the root.

Final mountpoints policy

As defined in https://github.com/osbuild/images/blob/main/internal/pathpolicy/policies.go