Keeping a copy on system-image.u.c will not help at all, as the IDs are assigned when building the live filesystem. We'd need the assignments to be actually in a package uploaded to the archive and installed in the image, and for that to be honoured by adduser --system, I think.
Keeping a copy on system-image.u.c will not help at all, as the IDs are assigned when building the live filesystem. We'd need the assignments to be actually in a package uploaded to the archive and installed in the image, and for that to be honoured by adduser --system, I think.