Create "make stage" for staging container

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

Makefile

@@ -13,3 +13,6 @@ build-preview: ## Build site with drafts and future posts enabled.
 
 serve: ## Boot the development server.
 	jekyll serve
+
+stage: ## Run the Jekyll staging container.
+	docker run -ti --rm -v "$PWD":/k8sdocs -p 4000:4000 gcr.io/google-samples/k8sdocs:1.1

docs/home/contribute/stage-documentation-changes.md

@@ -43,10 +43,13 @@ for this image.
 1. In the root of your cloned repository, enter this command to start a local
 web server:
 
+       make stage
+
+   This will run the following command:
+
        docker run -ti --rm -v "$PWD":/k8sdocs -p 4000:4000 gcr.io/google-samples/k8sdocs:1.1
 
-1. View your staged content at
+1. View your staged content at `http://localhost:4000`.
-http://localhost:4000.
 
 ## Staging locally without Docker
 
@@ -69,8 +72,7 @@ web server:
 
        jekyll serve
 
-1. View your staged content at
+1. View your staged content at `http://localhost:4000`.
-http://localhost:4000.
 
 <i>NOTE: If you do not want Jekyll to interfere with your other globally installed gems, you can use `bundler`:</i>