diff --git a/serving/samples/build-private-repo-go/README.md b/serving/samples/build-private-repo-go/README.md
index 499ed8d28..7476f2bd4 100644
--- a/serving/samples/build-private-repo-go/README.md
+++ b/serving/samples/build-private-repo-go/README.md
@@ -190,7 +190,10 @@ The sample code is in a private Github repository consisting of two files.
 
 1. `Dockerfile`
    ```Dockerfile
-   FROM golang
+   # Use golang:alpine to optimize the image size.
+   # See https://hub.docker.com/_/golang/ for more information
+   # about the difference between golang and golang:alpine.
+   FROM golang:alpine
 
    ENV GOPATH /go