Add link to image in Dockerhub (#747)

* Add link to image in Dockerhub

Mainly for consistency with other samples.

* also README

* Fixes indentation issue causing build test failure

* fix bullet list indentation

* attempt #2 -- fixing build failure

serving/samples/helloworld-go/Dockerfile

@@ -1,5 +1,6 @@
 # Use the offical Golang image to create a build artifact.
 # This is based on Debian and sets the GOPATH to /go.
+# https://hub.docker.com/_/golang
 FROM golang as builder
 
 # Copy local code to the container image.

serving/samples/helloworld-go/README.md

@@ -61,6 +61,7 @@ recreate the source files from this folder.
    ```docker
    # Use the offical Golang image to create a build artifact.
    # This is based on Debian and sets the GOPATH to /go.
+   # https://hub.docker.com/_/golang
    FROM golang as builder
 
    # Copy local code to the container image.
@@ -136,11 +137,10 @@ folder) you're ready to build and deploy the sample app.
    ```
 
 1. Now that your service is created, Knative will perform the following steps:
-
-   - Create a new immutable revision for this version of the app.
-   - Network programming to create a route, ingress, service, and load balance
-     for your app.
-   - Automatically scale your pods up and down (including to zero active pods).
+    - Create a new immutable revision for this version of the app.
+    - Network programming to create a route, ingress, service, and load balance
+    for your app.
+    - Automatically scale your pods up and down (including to zero active pods).
 
 1. Run the following command to find the external IP address for your service.
    The ingress IP for your cluster is returned. If you just created your