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The goal of this example is to show you how to get a Node.js application into aDocker container. The guide is intended for development, and not for aproduction deployment. The guide also assumes you have a working Dockerinstallation and a basicunderstanding of how a Node.js application is structured.
In the first part of this guide we will create a simple web application inNode.js, then we will build a Docker image for that application, and lastly wewill instantiate a container from that image.
Docker allows you to package an application with its environment and all of its dependencies into a'box', called a container. Usually, a container consists of an application running in a stripped-to-basics version of a Linux operating system. An image is the blueprint for a container, a container is a running instance of an image.
First, create a new directory where all the files would live. In this directorycreate a package.json
file that describes your app and its dependencies:
With your new package.json
file, run npm install
. If you are using npm
version 5 or later, this will generate a package-lock.json
file which will be copiedto your Docker image.
Then, create a server.js
file that defines a web app using theExpress.js framework:
In the next steps, we'll look at how you can run this app inside a Dockercontainer using the official Docker image. First, you'll need to build a Dockerimage of your app.
Create an empty file called 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 14
of node
available from the Docker Hub:
Next we create a directory to hold the application code inside the image, thiswill be the working directory for your application:
This image comes with Node.js and NPM already installed so the next thing weneed to do is to install your app dependencies using the npm
binary. Pleasenote that if you are using npm
version 4 or earlier a package-lock.json
file will not be generated.
Note that, rather than copying the entire working directory, we are only copyingthe package.json
file. This allows us to take advantage of cached Dockerlayers. bitJudo has a good explanation of thishere.Furthermore, the npm ci
command, specified in the comments, helps provide faster, reliable, reproducible builds for production environments.You can read more about this here.
To bundle your app's source code inside the Docker image, use the COPY
instruction:
Your app binds to port 8080
so you'll use the EXPOSE
instruction to have itmapped by the docker
daemon:
Last but not least, define the command to run your app using CMD
which definesyour runtime. Here we will use node server.js
to start your server:
Your Dockerfile
should now look like this:
Create a .dockerignore
file in the same directory as your Dockerfile
with following content:
This will prevent your local modules and debug logs from being copied onto yourDocker image and possibly overwriting modules installed within your image.
Go to the directory that has your Dockerfile
and run the following command tobuild the Docker image. The -t
flag lets you tag your image so it's easier tofind later using the docker images
command:
Your image will now be listed by Docker:
Running your image with -d
runs the container in detached mode, leaving thecontainer running in the background. The -p
flag redirects a public port to aprivate port inside the container. Run the image you previously built:
Print the output of your app:
If you need to go inside the container you can use the exec
command:
To test your app, get the port of your app that Docker mapped:
In the example above, Docker mapped the 8080
port inside of the container tothe port 49160
on your machine.
Now you can call your app using curl
(install if needed via: sudo apt-getinstall curl
):
We hope this tutorial helped you get up and running a simple Node.js applicationon Docker.
You can find more information about Docker and Node.js on Docker in thefollowing places: