docs: add documentation for creating user before boot

[sidalit]

Correct me if I'm wrong, currently, the Ubuntu Core documentation does not have a section to create a user at build time (i.e during image creation), the only documentation specifies how to add users once the system has booted (or during first boot).

Let me know if this is a valid suggestion and I would be happy to open a PR.

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Reported from: https://documentation.ubuntu.com/core/how-to-guides/manage-ubuntu-core/add-a-system-user/

[degville]

Hello! Thanks so much for these suggestions, and you're absolutely right about this. We don't currently document user-creation at build time, and what we do have on adding users afterwards is quite opaque.

I think having on user-creation would be a great addition to the documentation. I'll speak to the team about this, but if you do have a strategy that might work, please feel free to submit a PR. Thank you!

[sidalit]

Thanks @degville for taking into account this addition.

It seems that the addition is more a workaround than a definitive solution, here is the outcome for more context to speak to the team.

# After auto-import.assert is generated according to https://documentation.ubuntu.com/core/how-to-guides/manage-ubuntu-core/add-a-system-user/#creating-the-system-user-assertion

# Using pre-built image
$ sudo losetup --find --partscan --show <image>.img # </dev/loopXX>
$ sudo mount </dev/loopXXp1> /mnt
$ sudo cp auto-import.assert $(ls -1 -d /mnt/rootfs/root/systems/*)
$ sudo umount /mnt
$ sudo losetup -d </dev/loopXX>

# Using ubuntu-image
$ ubuntu-image snap <model> -w rootfs -t populate_rootfs_contents
$ cp auto-import.assert $(ls -1 -d rootfs/root/systems/*)
$ ubuntu-image snap <model> -w rootfs -r # generated image is located in rootfs/<image>.img

[degville]

here is the outcome for more context to speak to the team.

That's really useful, thank you. I'll speak to them today.