Update docker README with docker buildx instructions.

docker/README.md

@@ -30,7 +30,10 @@ make
 
 ## Build and push a multiarch build
 
-Make sure that `docker buildx` is supported.
+Make sure that `docker buildx` is supported (might be an experimental docker feature)
 ```bash
+BUILDER=$(docker buildx create --driver docker-container --use)
 docker buildx build --pull --push --platform linux/386,linux/amd64,linux/arm64,linux/arm/v7,linux/arm/v6 . -t chrislusf/seaweedfs
+docker buildx stop $BUILDER
 ```
+