The Docker platform allows developers to package and run applications as containers. A container is an isolated process that runs on a shared operating system, offering a lighter weight alternative to virtual machines. Though containers are not new, they offer benefits — including process isolation and environment standardization — that are growing in importance as more developers use distributed application architectures.
When building and scaling an application with Docker, the starting point is typically creating an image for your application, which you can then run in a container. The image includes your application code, libraries, configuration files, environment variables, and runtime. Using an image ensures that the environment in your container is standardized and contains only what is necessary to build and run your application.
In this tutorial, you will create an application image for a static website that uses the Express framework and Bootstrap. You will then build a container using that image and push it to Docker Hub for future use. Finally, you will pull the stored image from your Docker Hub repository and build another container, demonstrating how you can recreate and scale your application.
Prerequisites
To follow this tutorial, you will need:
Step 1 — Installing Your Application Dependencies
To create your image, you will first need to make your application files, which you can then copy to your container. These files will include your application’s static content, code, and dependencies.
First, create a directory for your project in your non-root user’s home directory. We will call ours node_project, but you should feel free to replace this with something else:
Navigate to this directory:
This will be the root directory of the project.
Next, create a package.json file with your project’s dependencies and other identifying information. Open the file with nano or your favorite editor:
Add the following information about the project, including its name, author, license, entrypoint, and dependencies. Be sure to replace the author information with your own name and contact details:
~/node_project/package.json
{
"name": "nodejs-image-demo",
"version": "1.0.0",
"description": "nodejs image demo",
"author": "Sammy the Shark <sammy@example.com>",
"license": "MIT",
"main": "app.js",
"keywords": [
"nodejs",
"bootstrap",
"express"
],
"dependencies": {
"express": "^4.16.4"
}
}
This file includes the project name, author, and license under which it is being shared. Npm recommends making your project name short and descriptive, and avoiding duplicates in the npm registry. We’ve listed the MIT license in the license field, permitting the free use and distribution of the application code.
Additionally, the file specifies:
"main": The entrypoint for the application, app.js. You will create this file next."dependencies": The project dependencies — in this case, Express 4.16.4 or above.
Though this file does not list a repository, you can add one by following these guidelines on adding a repository to your package.json file. This is a good addition if you are versioning your application.
Save and close the file when you’ve finished making changes.
To install your project’s dependencies, run the following command:
This will install the packages you’ve listed in your package.json file in your project directory.
We can now move on to building the application files.
Step 2 — Creating the Application Files
We will create a website that offers users information about sharks. Our application will have a main entrypoint, app.js, and a views directory that will include the project’s static assets. The landing page, index.html, will offer users some preliminary information and a link to a page with more detailed shark information, sharks.html. In the views directory, we will create both the landing page and sharks.html.
First, open app.js in the main project directory to define the project’s routes:
The first part of the file will create the Express application and Router objects, and define the base directory and port as constants:
~/node_project/app.js
const express = require('express');
const app = express();
const router = express.Router();
const path = __dirname + '/views/';
const port = 8080;
The require function loads the express module, which we then use to create the app and router objects. The router object will perform the routing function of the application, and as we define HTTP method routes we will add them to this object to define how our application will handle requests.
This section of the file also sets a couple of constants, path and port:
path: Defines the base directory, which will be the views subdirectory within the current project directory.port: Tells the app to listen on and bind to port 8080.
Next, set the routes for the application using the router object:
~/node_project/app.js
...
router.use(function (req,res,next) {
console.log('/' + req.method);
next();
});
router.get('/', function(req,res){
res.sendFile(path + 'index.html');
});
router.get('/sharks', function(req,res){
res.sendFile(path + 'sharks.html');
});
The router.use function loads a middleware function that will log the router’s requests and pass them on to the application’s routes. These are defined in the subsequent functions, which specify that a GET request to the base project URL should return the index.html page, while a GET request to the /sharks route should return sharks.html.
Finally, mount the router middleware and the application’s static assets and tell the app to listen on port 8080:
~/node_project/app.js
...
app.use(express.static(path));
app.use('/', router);
app.listen(port, function () {
console.log('Example app listening on port 8080!')
})
The finished app.js file will look like this:
~/node_project/app.js
const express = require('express');
const app = express();
const router = express.Router();
const path = __dirname + '/views/';
const port = 8080;
router.use(function (req,res,next) {
console.log('/' + req.method);
next();
});
router.get('/', function(req,res){
res.sendFile(path + 'index.html');
});
router.get('/sharks', function(req,res){
res.sendFile(path + 'sharks.html');
});
app.use(express.static(path));
app.use('/', router);
app.listen(port, function () {
console.log('Example app listening on port 8080!')
})
Save and close the file when you are finished.
Next, let’s add some static content to the application. Start by creating the views directory:
Open the landing page file, index.html:
Add the following code to the file, which will import Boostrap and create a jumbotron component with a link to the more detailed sharks.html info page:
~/node_project/views/index.html
<!DOCTYPE html>
<html lang="en">
<head>
<title>About Sharks</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
<link href="css/styles.css" rel="stylesheet">
<link href="https://fonts.googleapis.com/css?family=Merriweather:400,700" rel="stylesheet" type="text/css">
</head>
<body>
<nav class="navbar navbar-dark bg-dark navbar-static-top navbar-expand-md">
<div class="container">
<button type="button" class="navbar-toggler collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false"> <span class="sr-only">Toggle navigation</span>
</button> <a class="navbar-brand" href="#">Everything Sharks</a>
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav mr-auto">
<li class="active nav-item"><a href="/" class="nav-link">Home</a>
</li>
<li class="nav-item"><a href="/sharks" class="nav-link">Sharks</a>
</li>
</ul>
</div>
</div>
</nav>
<div class="jumbotron">
<div class="container">
<h1>Want to Learn About Sharks?</h1>
<p>Are you ready to learn about sharks?</p>
<br>
<p><a class="btn btn-primary btn-lg" href="/sharks" role="button">Get Shark Info</a>
</p>
</div>
</div>
<div class="container">
<div class="row">
<div class="col-lg-6">
<h3>Not all sharks are alike</h3>
<p>Though some are dangerous, sharks generally do not attack humans. Out of the 500 species known to researchers, only 30 have been known to attack humans.
</p>
</div>
<div class="col-lg-6">
<h3>Sharks are ancient</h3>
<p>There is evidence to suggest that sharks lived up to 400 million years ago.
</p>
</div>
</div>
</div>
</body>
</html>
The top-level navbar here allows users to toggle between the Home and Sharks pages. In the navbar-nav subcomponent, we are using Bootstrap’s active class to indicate the current page to the user. We’ve also specified the routes to our static pages, which match the routes we defined in app.js:
~/node_project/views/index.html
...
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav mr-auto">
<li class="active nav-item"><a href="/" class="nav-link">Home</a>
</li>
<li class="nav-item"><a href="/sharks" class="nav-link">Sharks</a>
</li>
</ul>
</div>
...
Additionally, we’ve created a link to our shark information page in our jumbotron’s button:
~/node_project/views/index.html
...
<div class="jumbotron">
<div class="container">
<h1>Want to Learn About Sharks?</h1>
<p>Are you ready to learn about sharks?</p>
<br>
<p><a class="btn btn-primary btn-lg" href="/sharks" role="button">Get Shark Info</a>
</p>
</div>
</div>
...
There is also a link to a custom style sheet in the header:
~/node_project/views/index.html
...
<link href="css/styles.css" rel="stylesheet">
...
We will create this style sheet at the end of this step.
Save and close the file when you are finished.
With the application landing page in place, we can create our shark information page, sharks.html, which will offer interested users more information about sharks.
Open the file:
Add the following code, which imports Bootstrap and the custom style sheet and offers users detailed information about certain sharks:
~/node_project/views/sharks.html
<!DOCTYPE html>
<html lang="en">
<head>
<title>About Sharks</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
<link href="css/styles.css" rel="stylesheet">
<link href="https://fonts.googleapis.com/css?family=Merriweather:400,700" rel="stylesheet" type="text/css">
</head>
<nav class="navbar navbar-dark bg-dark navbar-static-top navbar-expand-md">
<div class="container">
<button type="button" class="navbar-toggler collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false"> <span class="sr-only">Toggle navigation</span>
</button> <a class="navbar-brand" href="/">Everything Sharks</a>
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav mr-auto">
<li class="nav-item"><a href="/" class="nav-link">Home</a>
</li>
<li class="active nav-item"><a href="/sharks" class="nav-link">Sharks</a>
</li>
</ul>
</div>
</div>
</nav>
<div class="jumbotron text-center">
<h1>Shark Info</h1>
</div>
<div class="container">
<div class="row">
<div class="col-lg-6">
<p>
<div class="caption">Some sharks are known to be dangerous to humans, though many more are not. The sawshark, for example, is not considered a threat to humans.
</div>
<img src="https://assets.digitalocean.com/articles/docker_node_image/sawshark.jpg" alt="Sawshark">
</p>
</div>
<div class="col-lg-6">
<p>
<div class="caption">Other sharks are known to be friendly and welcoming!</div>
<img src="https://assets.digitalocean.com/articles/docker_node_image/sammy.png" alt="Sammy the Shark">
</p>
</div>
</div>
</div>
</html>
Note that in this file, we again use the active class to indicate the current page.
Save and close the file when you are finished.
Finally, create the custom CSS style sheet that you’ve linked to in index.html and sharks.html by first creating a css folder in the views directory:
Open the style sheet:
- nano views/css/styles.css
Add the following code, which will set the desired color and font for our pages:
~/node_project/views/css/styles.css
.navbar {
margin-bottom: 0;
}
body {
background: #020A1B;
color: #ffffff;
font-family: 'Merriweather', sans-serif;
}
h1,
h2 {
font-weight: bold;
}
p {
font-size: 16px;
color: #ffffff;
}
.jumbotron {
background: #0048CD;
color: white;
text-align: center;
}
.jumbotron p {
color: white;
font-size: 26px;
}
.btn-primary {
color: #fff;
text-color: #000000;
border-color: white;
margin-bottom: 5px;
}
img,
video,
audio {
margin-top: 20px;
max-width: 80%;
}
div.caption: {
float: left;
clear: both;
}
In addition to setting font and color, this file also limits the size of the images by specifying a max-width of 80%. This will prevent them from taking up more room than we would like on the page.
Save and close the file when you are finished.
With the application files in place and the project dependencies installed, you are ready to start the application.
If you followed the initial server setup tutorial in the prerequisites, you will have an active firewall permitting only SSH traffic. To permit traffic to port 8080 run:
To start the application, make sure that you are in your project’s root directory:
Start the application with node app.js:
Navigate your browser to http://your_server_ip:8080. You will see the following landing page:
Click on the Get Shark Info button. You will see the following information page:
You now have an application up and running. When you are ready, quit the server by typing CTRL+C. We can now move on to creating the Dockerfile that will allow us to recreate and scale this application as desired.
Step 3 — Writing the Dockerfile
Your Dockerfile specifies what will be included in your application container when it is executed. Using a Dockerfile allows you to define your container environment and avoid discrepancies with dependencies or runtime versions.
Following these guidelines on building optimized containers, we will make our image as efficient as possible by minimizing the number of image layers and restricting the image’s function to a single purpose — recreating our application files and static content.
In your project’s root directory, create the Dockerfile:
Docker images are created using a succession of layered images that build on one another. Our first step will be to add the base image for our application that will form the starting point of the application build.
Let’s use the node:10-alpine image, since at the time of writing this is the recommended LTS version of Node.js. The alpine image is derived from the Alpine Linux project, and will help us keep our image size down. For more information about whether or not the alpine image is the right choice for your project, please see the full discussion under the Image Variants section of the Docker Hub Node image page.
Add the following FROM instruction to set the application’s base image:
~/node_project/Dockerfile
FROM node:10-alpine
This image includes Node.js and npm. Each Dockerfile must begin with a FROM instruction.
By default, the Docker Node image includes a non-root node user that you can use to avoid running your application container as root. It is a recommended security practice to avoid running containers as root and to restrict capabilities within the container to only those required to run its processes. We will therefore use the node user’s home directory as the working directory for our application and set them as our user inside the container. For more information about best practices when working with the Docker Node image, see this best practices guide.
To fine-tune the permissions on our application code in the container, let’s create the node_modules subdirectory in /home/node along with the app directory. Creating these directories will ensure that they have the permissions we want, which will be important when we create local node modules in the container with npm install. In addition to creating these directories, we will set ownership on them to our node user:
~/node_project/Dockerfile
...
RUN mkdir -p /home/node/app/node_modules && chown -R node:node /home/node/app
For more information on the utility of consolidating RUN instructions, see this discussion of how to manage container layers.
Next, set the working directory of the application to /home/node/app:
~/node_project/Dockerfile
...
WORKDIR /home/node/app
If a WORKDIR isn’t set, Docker will create one by default, so it’s a good idea to set it explicitly.
Next, copy the package.json and package-lock.json (for npm 5+) files:
~/node_project/Dockerfile
...
COPY package*.json ./
Adding this COPY instruction before running npm install or copying the application code allows us to take advantage of Docker’s caching mechanism. At each stage in the build, Docker will check to see if it has a layer cached for that particular instruction. If we change package.json, this layer will be rebuilt, but if we don’t, this instruction will allow Docker to use the existing image layer and skip reinstalling our node modules.
To ensure that all of the application files are owned by the non-root node user, including the contents of the node_modules directory, switch the user to node before running npm install:
~/node_project/Dockerfile
...
USER node
After copying the project dependencies and switching our user, we can run npm install:
~/node_project/Dockerfile
...
RUN npm install
Next, copy your application code with the appropriate permissions to the application directory on the container:
~/node_project/Dockerfile
...
COPY --chown=node:node . .
This will ensure that the application files are owned by the non-root node user.
Finally, expose port 8080 on the container and start the application:
~/node_project/Dockerfile
...
EXPOSE 8080
CMD [ "node", "app.js" ]
EXPOSE does not publish the port, but instead functions as a way of documenting which ports on the container will be published at runtime. CMD runs the command to start the application — in this case, node app.js. Note that there should only be one CMD instruction in each Dockerfile. If you include more than one, only the last will take effect.
There are many things you can do with the Dockerfile. For a complete list of instructions, please refer to Docker’s Dockerfile reference documentation.
The complete Dockerfile looks like this:
~/node_project/Dockerfile
FROM node:10-alpine
RUN mkdir -p /home/node/app/node_modules && chown -R node:node /home/node/app
WORKDIR /home/node/app
COPY package*.json ./
USER node
RUN npm install
COPY --chown=node:node . .
EXPOSE 8080
CMD [ "node", "app.js" ]
Save and close the file when you are finished editing.
Before building the application image, let’s add a .dockerignore file. Working in a similar way to a .gitignore file, .dockerignore specifies which files and directories in your project directory should not be copied over to your container.
Open the .dockerignore file:
Inside the file, add your local node modules, npm logs, Dockerfile, and .dockerignore file:
~/node_project/.dockerignore
node_modules
npm-debug.log
Dockerfile
.dockerignore
If you are working with Git then you will also want to add your .git directory and .gitignore file.
Save and close the file when you are finished.
You are now ready to build the application image using the docker build command. Using the -t flag with docker build will allow you to tag the image with a memorable name. Because we are going to push the image to Docker Hub, let’s include our Docker Hub username in the tag. We will tag the image as nodejs-image-demo, but feel free to replace this with a name of your own choosing. Remember to also replace your_dockerhub_username with your own Docker Hub username:
- docker build -t your_dockerhub_username/nodejs-image-demo .
The . specifies that the build context is the current directory.
It will take a minute or two to build the image. Once it is complete, check your images:
You will see the following output:
Output
REPOSITORY TAG IMAGE ID CREATED SIZE
your_dockerhub_username/nodejs-image-demo latest 1c723fb2ef12 8 seconds ago 73MB
node 10-alpine f09e7c96b6de 3 weeks ago 70.7MB
It is now possible to create a container with this image using docker run. We will include three flags with this command:
-p: This publishes the port on the container and maps it to a port on our host. We will use port 80 on the host, but you should feel free to modify this as necessary if you have another process running on that port. For more information about how this works, see this discussion in the Docker docs on port binding.-d: This runs the container in the background.--name: This allows us to give the container a memorable name.
Run the following command to build the container:
- docker run --name nodejs-image-demo -p 80:8080 -d your_dockerhub_username/nodejs-image-demo
Once your container is up and running, you can inspect a list of your running containers with docker ps:
You will see the following output:
Output
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
e50ad27074a7 your_dockerhub_username/nodejs-image-demo "node app.js" 8 seconds ago Up 7 seconds 0.0.0.0:80->8080/tcp nodejs-image-demo
With your container running, you can now visit your application by navigating your browser to http://your_server_ip. You will see your application landing page once again:
Now that you have created an image for your application, you can push it to Docker Hub for future use.
Step 4 — Using a Repository to Work with Images
By pushing your application image to a registry like Docker Hub, you make it available for subsequent use as you build and scale your containers. We will demonstrate how this works by pushing the application image to a repository and then using the image to recreate our container.
The first step to pushing the image is to log in to the Docker Hub account you created in the prerequisites:
- docker login -u your_dockerhub_username
When prompted, enter your Docker Hub account password. Logging in this way will create a ~/.docker/config.json file in your user’s home directory with your Docker Hub credentials.
You can now push the application image to Docker Hub using the tag you created earlier, your_dockerhub_username/nodejs-image-demo:
- docker push your_dockerhub_username/nodejs-image-demo
Let’s test the utility of the image registry by destroying our current application container and image and rebuilding them with the image in our repository.
First, list your running containers:
You will see the following output:
Output
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
e50ad27074a7 your_dockerhub_username/nodejs-image-demo "node app.js" 3 minutes ago Up 3 minutes 0.0.0.0:80->8080/tcp nodejs-image-demo
Using the CONTAINER ID listed in your output, stop the running application container. Be sure to replace the highlighted ID below with your own CONTAINER ID:
List your all of your images with the -a flag:
You will see the following output with the name of your image, your_dockerhub_username/nodejs-image-demo, along with the node image and the other images from your build:
Output
REPOSITORY TAG IMAGE ID CREATED SIZE
your_dockerhub_username/nodejs-image-demo latest 1c723fb2ef12 7 minutes ago 73MB
<none> <none> 2e3267d9ac02 4 minutes ago 72.9MB
<none> <none> 8352b41730b9 4 minutes ago 73MB
<none> <none> 5d58b92823cb 4 minutes ago 73MB
<none> <none> 3f1e35d7062a 4 minutes ago 73MB
<none> <none> 02176311e4d0 4 minutes ago 73MB
<none> <none> 8e84b33edcda 4 minutes ago 70.7MB
<none> <none> 6a5ed70f86f2 4 minutes ago 70.7MB
<none> <none> 776b2637d3c1 4 minutes ago 70.7MB
node 10-alpine f09e7c96b6de 3 weeks ago 70.7MB
Remove the stopped container and all of the images, including unused or dangling images, with the following command:
Type y when prompted in the output to confirm that you would like to remove the stopped container and images. Be advised that this will also remove your build cache.
You have now removed both the container running your application image and the image itself. For more information on removing Docker containers, images, and volumes, please see How To Remove Docker Images, Containers, and Volumes.
With all of your images and containers deleted, you can now pull the application image from Docker Hub:
- docker pull your_dockerhub_username/nodejs-image-demo
List your images once again:
You will see your application image:
Output
REPOSITORY TAG IMAGE ID CREATED SIZE
your_dockerhub_username/nodejs-image-demo latest 1c723fb2ef12 11 minutes ago 73MB
You can now rebuild your container using the command from Step 3:
- docker run --name nodejs-image-demo -p 80:8080 -d your_dockerhub_username/nodejs-image-demo
List your running containers:
Output
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
f6bc2f50dff6 your_dockerhub_username/nodejs-image-demo "node app.js" 4 seconds ago Up 3 seconds 0.0.0.0:80->8080/tcp nodejs-image-demo
Visit http://your_server_ip once again to view your running application.
Conclusion
In this tutorial you created a static web application with Express and Bootstrap, as well as a Docker image for this application. You used this image to create a container and pushed the image to Docker Hub. From there, you were able to destroy your image and container and recreate them using your Docker Hub repository.
