image-builder: document the LILO re-run rule in fstab.example

The docker host boots BIOS/legacy with LILO in the MBR; GRUB is installed
but only as EFI binaries the firmware never loads, so it is inert rather
than competing. Worth writing down, because "two bootloaders installed"
reads as ambiguous until you check which one the firmware actually uses.

The part that bites is that LILO maps the kernel by physical block, so
`lilo -v` has to be re-run by hand after a kernel or initrd change. Skip
it and the machine looks fine until it reboots, which on a headless VM
means recovering from the console.

Noted next to the mount entries, and noted that editing those entries
specifically does not need it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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