syncucer will only write witrh syncuser switch

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

internal/app/wwctl/container/syncuser/root.go

@@ -24,11 +24,11 @@ uid/gid collision is detected. File ownerships are also changed.`,
 
 		Args: cobra.MinimumNArgs(1),
 	}
-	noSyncUser bool
+	write bool
 )
 
 func init() {
-	baseCmd.PersistentFlags().BoolVar(&noSyncUser, "nosyncuser", false, "Don't synchronize uis/gods just check")
+	baseCmd.PersistentFlags().BoolVar(&write, "write", false, "Synchronize uis/gids and write files in container")
 }
 
 // GetRootCommand returns the root cobra.Command for the application.