It is a simple hello world application to get to know about docker and nodejs. In the first part, we will create a simple nodejs web application and then we will build a docker image for the web application and lastly we will push the docker image on dockerhub.

Make sure to have node.js installed on your system. If not, install it from https://nodejs.org/en/download/package-manager/

Create a server.js file and insert the below given code in it.

var express = require('express');
var app = express();

app.get('/', function (req, res) {
  res.send('Hello World!');
});

app.listen(3000, function () {
  console.log('Example app listening on port 3000!');
});

Let's try to understand what we are trying to do with these sets of instructions.

• First 2 lines instruct the nodejs application that it requires express module and initialise it in app variable.
• Next 3 lines takes GET request at '/' path and reponds with "Hello World!".
• In the last 3 lines, app listens on port 3000 for connections.

Now let's initialize package.json to list all the dependencies needed for the server to run.

{
  "name": "docker_helloworld",
  "version": "1.0.0",
  "description": "Docker Node.js hello world web application",
  "author": "First-name last-name <email>",
  "dependencies": {
    "express": "^4.13.4"
  }
}

In the package.json, we have listed express module and its latest version. Name, version, description and author are optional parameters. Now let us install the dependencies using

$ npm install

Finally, start the server using

$ node server.js

Open localhost:3000 on your browser. Here you will get a "Hello world" message.

We have created our node.js hello world application. Now let's try to dockerize it.

Let's create a docker file and save it as Dockerfile to build an image of our hello world application.

FROM	ubuntu:14.04
RUN 	apt-get update && apt-get install -y curl && apt-get clean && rm -rf /var/lib/apt/lists
RUN		curl -sL https://deb.nodesource.com/setup_5.x | sudo -E bash -
RUN		apt-get update && apt-get install -y nodejs 

COPY	package.json /src/package.json
RUN		cd /src; npm install --production
COPY	. /src

EXPOSE  3000
CMD		["node", "/src/server.js"]

In this Dockerfile,

• First line, docker uses ubuntu 14:04 as the operating system
• Second line, it installs curl
• 3rd and 4th line install nodejs with version 5.x (Instead of first 3 lines, one can also use a pre-built image of nodejs. For that replace first 3 lines with: FROM shubhag/nodejs)
• Next line copy package.json file into docker image
• 7th line, runs npm install to install the node dependencies.
• Next line copies the source server file into docker images (. implies everything in the current folder)
• Last 2 lines, expose port 3000 and specify the command to run nodejs server.

Now, let's build this image using

$ sudo docker build -t hello-world .

-t parameter here specifies the name for the image (you can give any name)

Finally to run the docker container using the previously build image, use

$ sudo docker run -p 3000:3000 -d --name server hello-world

Now, open localhost:3000 in your browser. Voila, you have dockerized your node.js hello world application.

Docker allows you to push your images on dockerhub which then can be later used. The main advantage is if you want to run your dockerized helloworld or any other application and you do not have the docker image of it present on your system, then you can just pull that image from dockerhub and run your application without bothering about building that image again.

In order to push an image on dockerhub, signup for dockerhub. After signup, login to dockerhub using your login credentials.

$ sudo docker login

Now, we have to re-tag our hello-world image in order to include username(dockerhub username) as well in the tag.

$ sudo docker tag hello-world <username>/hello-world

Finally, push the image using

$ sudo docker push <username>/hello-world