Serving a node application in Docker is actually fairly simply. In this post, we’ll do so using only a six-line Dockerfile!

Create a Sample App

For our proof-of-concept, we’ll create a sample node app in a new directory. Let’s quickly put one together.

Create app directory

First create a node-in-docker directory and cd into it.

mkdir node-in-docker
cd node-in-docker

initialize npm project with defaults

Next, initialize an npm project. The -y flag will initilize with default settings.

npm init -y

Add express.js and simple code

Our app will be an express app.

npm i express

Create an index.js file with some simple home route handling.

touch index.js

With the following code:

const express = require('express');
const app = express();
const port = process.env.NODE_SERVER_PORT;

app.get('/', (req, res) => res.send('Sample node app!'));

app.listen(port, () =>
  console.log(`Example app listening at http://localhost:${port}`)
);

Note that our port will come from a NODE_SERVER_PORT environment variable.

Test the app

Let’s test that our app looks good before we add any Docker code. We can do this by using node . and remembering to include our environment variable.

NODE_SERVER_PORT=3000 node .

Now if we navigate to http://localhost:3000, we should see our sample app.

If you see this, we’re now ready to dive into docker land!

Add Docker

In this minimal example, all we’ll need is two files:

1. Dockerfile
2. .dockerignore

The Dockerfile will be the blueprint for creating our Docker image and the .dockerignore will be used to tell Docker to ignore the node_modules folder when building the image.

.dockerignore

The .dockerignore file is pretty easy, let’s just throw the string node_modules in there.

echo "node_modules" > .dockerignore

Dockerfile

The Dockerfile is a bit more complex, but not by much!

Let’s first create the file

touch Dockerfile

And now here’s the magic. Read the comments to see what’s going on.

# Use the node v10 image
FROM node:10
# Set working directory
WORKDIR /app
# Copy contents of current directory into working dir
COPY . .
# install node modules
RUN npm i
# Set the node server port to 3000
ENV NODE_SERVER_PORT=3000
# Containers start the app by running node .
ENTRYPOINT ["node", "."]

Build the Docker image

Let’s build the docker image based on the instructions in our Dockerfile!

We will use the tag (-t) node-server and our build context will be the current directory (.).

docker build -t node-server .

Run a container

Now we can create and run a container based on our node-server image! To so so, we’ll use the following options:

• --rm to cleanup old instances
• -it flags for terminal behavior
• Expose container port 3000 on port 3000

docker run --rm -it -p 3000:3000 node-server

Now if everything works, we should be able to go to http://localhost:3000 and see our sample node app!