Change default userId because of Debian 13 changes

[Korbeil]

Updates the fallback user ID used in the Castor Docker context from 1000 to 999 when the current user ID cannot be determined or is invalid (e.g. running as root).

Motivation

On Debian (and many Linux systems), UIDs 1000 and above are reserved for regular (“normal”) users. UIDs below 1000 are typically used for system users. Using 1000 for a system/container user triggers a warning on Debian because it overlaps with the range intended for human users.
Using 999 keeps the fallback in the system-user range and avoids that warning while remaining a safe, non-root value.

Here is the output with Debian 13 when using user ID 1000

useradd warning: app's uid 1000 is greater than SYS_UID_MAX 999

[joelwurtz]

Not sure we want that ?

It should still be 1000 because we want it to match the user id of the person creating the container. It's a warning because, yes user usually start with 1000 as uid, but it's wanted, otherwise there is permissions issue and it's a ****** nightmare.

[lyrixx]

Not sure we want that ?

I agree with @joelwurtz. 999 is system user, and here we don't want a system user...

Thanks for the report, but I'm -1 with this one.

More over, you changed the fallback, but in 99%, this code isn't even reached. We use your UID (aka 1000).

So this patch may not fix the issue you got