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/pkg/container/syncuids.go

@@ -100,7 +100,8 @@ func SyncUids(containerName string, showOnly bool) error {
 		*/
 
 	}
-	if !showOnly {
+	if showOnly {
+		wwlog.Printf(wwlog.INFO, "uid./gid not synced, run \nwwctl container syncuser --write %s\nto synchronize uid/gids.\n", containerName)
 		return nil
 	}
 	// create list of files which need changed ownerships in order to change them later what