uid/gid are not synced at container import but checked. An apropriate
message is printed, regarding users which are not present on host.
The `wwctl container syncuser` needs the `--write` switch to actually
write to the container.
The command `wwctl container syncuser` will always operate at the
container level, but as the 'generic` overlay reads in the container
passwd/group, following two steps will update the passwd/group on a
running cluster:
1. Add passwd/group to container
wwctl container syncuser --write CONTAINER
2. Synchronize overlay
wwctl overlay build -N
