Troubles extending Ubuntu 22.04 ISO

[Ash Arnold]

I'm trying to add some files to the Ubuntu live install ISO that they 
provide, in order to customize the OS install a bit.


Starting iso is: 
https://releases.ubuntu.com/22.04.4/ubuntu-22.04.4-live-server-amd64.iso

I've got a directory full of stuff I want to add to it. (autoinstall, 
some debs, etc.)

And in a few places I saw this as the suggested command to insert new 
files while keeping the bootability of the existing ISO:

`xorriso -indev ~/Downloads/ubuntu-22.04.4-live-server-amd64.iso -outdev 
build/autoinstall-22.04.4.iso -map build/iso_modified / -boot_image any 
replay -compliance no_emul_toc -padding included`


But doing so gives an error with grub I don't understand:

GNU xorriso 1.5.6.pl02 : RockRidge filesystem manipulator, libburnia 
project.

xorriso : NOTE : Loading ISO image tree from LBA 0
xorriso : UPDATE :     820 nodes read in 1 seconds
libisofs: NOTE : Found hidden El-Torito image for EFI.
libisofs: NOTE : EFI image start and size: 1024860 * 2048 , 10068 * 512
xorriso : NOTE : Detected El-Torito boot information which currently is 
set to be discarded
Drive current: -indev 
'/home/ash/Downloads/ubuntu-22.04.4-live-server-amd64.iso'
Media current: stdio file, overwriteable
Media status : is written , is appendable
Boot record  : El Torito , MBR protective-msdos-label grub2-mbr 
cyl-align-off GPT
Media summary: 1 session, 1027543 data blocks, 2007m data,  830g free
Volume id    : 'Ubuntu-Server 22.04.4 LTS amd64'
Drive current: -outdev 'build/autoinstall-22.04.4.iso'
Media current: stdio file, overwriteable
Media status : is blank
Media summary: 0 sessions, 0 data blocks, 0 data,  830g free
xorriso : UPDATE :     962 files added in 1 seconds
Added to ISO image: directory 
'/'='/home/ash/src/ubuntu-autoinstall/build/iso-modified'
xorriso : SORRY : Cannot enable EL Torito boot image #1 because it is 
not a data file in the ISO filesystem
xorriso : WARNING : -volid text problematic as automatic mount point name
xorriso : WARNING : -volid text does not comply to ISO 9660 / ECMA 119 rules
xorriso : NOTE : Replayed 17 boot related commands
xorriso : NOTE : Copying to System Area: 32768 bytes from file 
'--interval:imported_iso:0s-15s:zero_mbrpt,zero_gpt:/home/ash/Downloads/ubuntu-22.04.4-live-server-amd64.iso'
libisofs: FAILURE : Cannot refer by GRUB2 MBR to data outside of ISO 
9660 filesystem.
libisofs: MISHAP : Image write cancelled
libburn : FAILURE : Premature end of input encountered. Missing: 
4553959424 bytes
xorriso : FAILURE : libburn indicates failure with writing.
xorriso : NOTE : -return_with SORRY 32 triggered by problem severity FAILURE

Some details that might help:

- I don't care about supporting MBR at all. Every system I want to image 
is using UEFI.

- I am making this for eventual install via USB boot: Is going through 
ISO a good route, or should I just format the stick, grub-install it, 
and copy files into it? (I don't understand entirely why the ISO has 3 
filesystems in it.)