cmd/sync-containers: add a dry-run option.

Updates tailscale/corp#8461

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/sync-containers/main.go

@@ -34,6 +34,7 @@ var (
 	src    = flag.String("src", "", "Source image")
 	dst    = flag.String("dst", "", "Destination image")
 	max    = flag.Int("max", 0, "Maximum number of tags to sync (0 for all tags)")
+	dryRun = flag.Bool("dry-run", true, "Don't actually sync anything")
 )
 
 func main() {
@@ -69,10 +70,14 @@ func main() {
 		}
 	}
 	for _, tag := range add {
+		if !*dryRun {
 			log.Printf("Syncing tag %q", tag)
 			if err := copyTag(*src, *dst, tag, opts...); err != nil {
 				log.Printf("Syncing tag %q: progress error: %v", tag, err)
 			}
+		} else {
+			log.Printf("Dry run: would sync tag %q", tag)
+		}
 	}
 
 	if len(remove) > 0 {