serving/samples/helloworld-nodejs: simplify and standardize Dockerfile (#507)

serving/samples/helloworld-nodejs/Dockerfile

@@ -1,17 +1,24 @@
+# Use the official Node 8 image.
+# https://hub.docker.com/_/node
 FROM node:8
 
-# Create app directory
+# Create and change to the app directory.
 WORKDIR /usr/src/app
 
-# Install app dependencies
-# A wildcard is used to ensure both package.json AND package-lock.json are copied
-# where available (npm@5+)
+# Copy application dependency manifests to the container image.
+# A wildcard is used to ensure both package.json AND package-lock.json are copied.
+# Copying this separately prevents re-running npm install on every code change.
 COPY package*.json ./
 
+# Install production dependencies.
 RUN npm install --only=production
 
-# Bundle app source
+# Copy local code to the container image.
 COPY . .
 
-EXPOSE 8080
+# Configure and document the service HTTP port.
+ENV PORT 8080
+EXPOSE $PORT
+
+# Run the web service on container startup.
 CMD [ "npm", "start" ]

serving/samples/helloworld-nodejs/README.md

@@ -83,22 +83,29 @@ recreate the source files from this folder.
    see [Dockerizing a Node.js web app](https://nodejs.org/en/docs/guides/nodejs-docker-webapp/).
 
     ```Dockerfile
+    # Use the official Node 8 image.
+    # https://hub.docker.com/_/node
     FROM node:8
 
-    # Create app directory
+    # Create and change to the app directory.
     WORKDIR /usr/src/app
 
-    # Install app dependencies
-    # A wildcard is used to ensure both package.json AND package-lock.json are copied
-    # where available (npm@5+)
+    # Copy application dependency manifests to the container image.
+    # A wildcard is used to ensure both package.json AND package-lock.json are copied.
+    # Copying this separately prevents re-running npm install on every code change.
     COPY package*.json ./
 
+    # Install production dependencies.
     RUN npm install --only=production
 
-    # Bundle app source
+    # Copy local code to the container image.
     COPY . .
 
-    EXPOSE 8080
+    # Configure and document the service HTTP port.
+    ENV PORT 8080
+    EXPOSE $PORT
+
+    # Run the web service on container startup.
     CMD [ "npm", "start" ]
     ```