refactor: don't set global loggers anymore

Ref #864

You were right @kotx, it is a bad idea to set the global logger
instance.

Signed-off-by: Xe Iaso <me@xeiaso.net>

cmd/containerbuild/main.go

@@ -28,7 +28,7 @@ func main() {
 	flagenv.Parse()
 	flag.Parse()
 
-	internal.InitSlog(*slogLevel)
+	slog.SetDefault(internal.InitSlog(*slogLevel, os.Stderr))
 
 	koDockerRepo := strings.TrimSuffix(*dockerRepo, "/"+filepath.Base(*dockerRepo))