Update README.md

Author: uji

docker/README.md

@@ -13,7 +13,7 @@ You can build docker image from source code locally.
 
     git clone https://github.com/antlr/antlr4.git
     cd antlr4/docker
-    docker build -t antlr/antlr4 .    
+    docker build -t antlr/antlr4 --platfort linux/amd64　.    
 
 
 ## Run