Use net.JoinHostPort to handle address format.

Signed-off-by: Dong Chen <dongluo.chen@docker.com>

pkg/discovery/backends.go

@@ -89,7 +89,7 @@ func ParseAdvertise(advertise string) (string, error) {
 		return "", fmt.Errorf("couldnt find a valid ip-address in interface %s", advertise)
 	}
 
-	addr = fmt.Sprintf("%s:%s", addr, port)
+	addr = net.JoinHostPort(addr, port)
 	return addr, nil
 }