Dockerizing a Node.js web app

The goal of this example is to show you how to get a Node.js application into a Docker container. The guide is intended for development, and not for a production deployment. The guide also assumes you have a working Docker installation and a basic understanding of how a Node.js application is structured.

In the first part of this guide we will create a simple web application in Node.js, then we will build a Docker image for that application, and lastly we will run the image as a container.

Docker allows you to package an application with all of its dependencies into a standardized unit, called a container, for software development. A container is a stripped-to-basics version of a Linux operating system. An image is software you load into a container.

Create the Node.js app

First, create a new directory where all the files would live. In this directory create apackage.json file that describes your app and its dependencies:

{
  "name": "docker_web_app",
  "version": "1.0.0",
  "description": "Node.js on Docker",
  "author": "First Last <first.last@example.com>",
  "main": "server.js",
  "scripts": {
    "start": "node server.js"
  },
  "dependencies": {
    "express": "^4.13.3"
  }
}

Then, create a server.js file that defines a web app using the Express.js framework:

'use strict';

const express = require('express');

// Constants
const PORT = 8080;

// App
const app = express();
app.get('/', function (req, res) {
  res.send('Hello world\n');
});

app.listen(PORT);
console.log('Running on http://localhost:' + PORT);

In the next steps, we'll look at how you can run this app inside a Docker container using the official Docker image. First, you'll need to build a Docker image of your app.

Creating a Dockerfile

Create an empty file called Dockerfile:

touch Dockerfile

Open the Dockerfile in your favorite text editor

The first thing we need to do is define from what image we want to build from. Here we will use the latest LTS (long term support) version argon of node available from the Docker Hub:

FROM node:argon

Next we create a directory to hold the application code inside the image, this will be the working directory for your application:

# Create app directory
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

This image comes with Node.js and NPM already installed so the next thing we need to do is to install your app dependencies using the npm binary:

# Install app dependencies
COPY package.json /usr/src/app/
RUN npm install

To bundle your app's source code inside the Docker image, use the COPY instruction:

# Bundle app source
COPY . /usr/src/app

Your app binds to port 8080 so you'll use the EXPOSE instruction to have it mapped by thedocker daemon:

EXPOSE 8080

Last but not least, define the command to run your app using CMD which defines your runtime. Here we will use the basic npm start which will run node server.js to start your server:

CMD [ "npm", "start" ]

Your Dockerfile should now look like this:

FROM node:argon

# Create app directory
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

# Install app dependencies
COPY package.json /usr/src/app/
RUN npm install

# Bundle app source
COPY . /usr/src/app

EXPOSE 8080
CMD [ "npm", "start" ]

Building your image

Go to the directory that has your Dockerfile and run the following command to build the Docker image. The -t flag lets you tag your image so it's easier to find later using thedocker images command:

$ docker build -t <your username>/node-web-app .

Your image will now be listed by Docker:

$ docker images

# Example
REPOSITORY                      TAG        ID              CREATED
node                            argon      539c0211cd76    3 weeks ago
<your username>/node-web-app    latest     d64d3505b0d2    1 minute ago

Run the image

Running your image with -d runs the container in detached mode, leaving the container running in the background. The -p flag redirects a public port to a private port inside the container. Run the image you previously built:

$ docker run -p 49160:8080 -d <your username>/node-web-app

Print the output of your app:

# Get container ID
$ docker ps

# Print app output
$ docker logs <container id>

# Example
Running on http://localhost:8080

If you need to go inside the container you can use the exec command:

# Enter the container
$ docker exec -it <container id> /bin/bash

Test

To test your app, get the port of your app that Docker mapped:

$ docker ps

# Example
ID            IMAGE                                COMMAND    ...   PORTS
ecce33b30ebf  <your username>/node-web-app:latest  npm start  ...   49160->8080

In the example above, Docker mapped the 8080 port inside of the container to the port49160 on your machine.

Now you can call your app using curl (install if needed via: sudo apt-get install curl):

$ curl -i localhost:49160

HTTP/1.1 200 OK
X-Powered-By: Express
Content-Type: text/html; charset=utf-8
Content-Length: 12
Date: Sun, 02 Jun 2013 03:53:22 GMT
Connection: keep-alive

Hello world

We hope this tutorial helped you get up and running a simple Node.js application on Docker.

You can find more information about Docker and Node.js on Docker in the following places:

• Official Node.js Docker Image
• Node.js Docker Best Practices Guide
• Official Docker documentation
• Docker Tag on StackOverflow
• Docker Subreddit