serving/samples/helloworld-go: simplify and standardize Dockerfile (#508)

serving/samples/helloworld-go/Dockerfile

@@ -1,17 +1,25 @@
-# Start from a Debian image with the latest version of Go installed
-# and a workspace (GOPATH) configured at /go.
-FROM golang
+# Use the offical Golang image to create a build artifact.
+# This is based on Debian and sets the GOPATH to /go.
+FROM golang as builder
 
-# Copy the local package files to the container's workspace.
-ADD . /go/src/github.com/knative/docs/helloworld
+# Copy local code to the container image.
+COPY . /go/src/github.com/knative/docs/helloworld
 
 # Build the outyet command inside the container.
 # (You may fetch or manage dependencies here,
 # either manually or with a tool like "godep".)
 RUN go install github.com/knative/docs/helloworld
 
-# Run the outyet command by default when the container starts.
-ENTRYPOINT ["/go/bin/helloworld"]
+# Use a Docker multi-stage build to create a lean production image.
+# https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
+FROM alpine
 
-# Document that the service listens on port 8080.
-EXPOSE 8080
+# Copy the binary to the production image from the builder stage.
+COPY --from=builder /go/bin/helloworld /helloworld
+
+# Configure and document the service HTTP port.
+ENV PORT 8080
+EXPOSE $PORT
+
+# Run the web service on container startup.
+CMD ["/helloworld"]

serving/samples/helloworld-go/README.md

@@ -59,23 +59,31 @@ following instructions recreate the source files from this folder.
    [Deploying Go servers with Docker](https://blog.golang.org/docker).
 
     ```docker
-    # Start from a Debian image with the latest version of Go installed
-    # and a workspace (GOPATH) configured at /go.
-    FROM golang
+    # Use the offical Golang image to create a build artifact.
+    # This is based on Debian and sets the GOPATH to /go.
+    FROM golang as builder
 
-    # Copy the local package files to the container's workspace.
-    ADD . /go/src/github.com/knative/docs/helloworld
+    # Copy local code to the container image.
+    COPY . /go/src/github.com/knative/docs/helloworld
 
-    # Build the helloworld command inside the container.
+    # Build the outyet command inside the container.
     # (You may fetch or manage dependencies here,
     # either manually or with a tool like "godep".)
     RUN go install github.com/knative/docs/helloworld
 
-    # Run the helloworld command by default when the container starts.
-    ENTRYPOINT /go/bin/helloworld
+    # Use a Docker multi-stage build to create a lean production image.
+    # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
+    FROM alpine
 
-    # Document that the service listens on port 8080.
-    EXPOSE 8080
+    # Copy the binary to the production image from the builder stage.
+    COPY --from=builder /go/bin/helloworld /helloworld
+
+    # Configure and document the service HTTP port.
+    ENV PORT 8080
+    EXPOSE $PORT
+
+    # Run the web service on container startup.
+    CMD ["/helloworld"]
     ```
 
 1. Create a new file, `service.yaml` and copy the following service definition