Docker

Build a Docker image of a Node.js application

Eric Cabrel TIOGO

• 5 min read
Build a Docker image of a Node.js application
Photo by eberhard 🖐 grossgasteiger / Unsplash

Photo by eberhard 🖐 grossgasteiger / Unsplash

For deploying a Node.js application in production, you can execute the code directly and use a process manager like PM2 to monitor your application. A better approach is to create a Docker image and start a container from that image.

With Docker, you can tag your image and revert to a previous working image when the current has a critical bug; developers can run the application without worrying about the OS and configuration settings.

We will see how to build a Docker image of a Node.js application and if you are interested in how to run the application with PM2, I wrote a post about it.

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Prerequisites

You need these tools installed on your computer to follow this tutorial.

Setup the project

We will use a Node.js Rest API we build in this previous article below.

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Let's clone the project locally:


git clone https://github.com/tericcabrel/blog-tutorials 

cd blog-tutorials/node-rest-api-swagger

You can follow the instructions in README to run the project locally. We will run the application after we build the Docker image.

Write the Dockerfile

We will take advantage of the Docker multi-stage build to:

  • Make the image build agnostic from any operating system
  • Reduce the size of the Docker image

At the root project directory, create a file called Dockerfile and add the code below:


FROM mhart/alpine-node:16 as builder

RUN mkdir -p /app

WORKDIR /app

COPY . .

RUN yarn install

RUN yarn tsc

FROM mhart/alpine-node:16 as app

ENV NODE_ENV=production

RUN mkdir -p /app

WORKDIR /app

COPY --chown=node:node --from=builder /app/package.json /app
COPY --chown=node:node --from=builder /app/build/ /app

RUN yarn install --frozen-lockfile --production

EXPOSE 4500

ENTRYPOINT ["node", "index.js"]

This file has two stages builder and app and use Node alpine 16 as the base image.

In the first stage builder, we copy the project file from the host, install the dependencies and build and transpile the files from Typescript to JavaScript.

In the second stage app, we copy the package.json and the content of the folder generated by the project built in the previous stage. We also set the ownership of the files to the user node (automatically created in the Node alpine image). Finally, we define the command to run when the container starts.

Run the command below to build the image:


docker build -t node-app .

View the image detail by running docker image ls.

Test the Docker image

Let's try to run the image and verify the application work as expected. Since the Node project interacts with MongoDB, so we need to start a container with the command below:


docker network create node-app-network

docker run -d --network node-app-network -e MONGO_INITDB_ROOT_USERNAME=app_user -e MONGO_INITDB_ROOT_PASSWORD=app_password  --name mongodb mongo:5.0

Create a file .env that will contain the environment variables to inject when starting a Docker container of our application image.


HOST=http://localhost
PORT=4500

DB_HOST=mongodb
DB_PORT=27017
DB_USER=app_user
DB_PASS=app_password
DB_NAME=test

Run the command below to start the container of the project


docker run -it -p 4500:4500 --network node-app-network --name node-rest-api --rm --env-file .env node-app:latest

Open your browser and navigate to http://localhost:4500/documentation

Reduce the size using Esbuild

After building the application, we still need the production dependencies to run the application, which is why the Docker image contains a node_modules folder.

I use a tool called Dive to explore the content of a Docker image. We can see the node_modules folder takes 191MB of space.

We can reduce the size if we package the application into a single file using a Javascript bundler like ESbuild.

Here is what the Dockerfile now looks like:


FROM mhart/alpine-node:16 as builder

RUN mkdir -p /app

WORKDIR /app

COPY . .

RUN yarn install

RUN npx esbuild ./src/index.ts --bundle --platform=node --outfile=build/index.js

FROM mhart/alpine-node:16 as app

ENV NODE_ENV=production

RUN mkdir -p /app

WORKDIR /app

COPY --chown=node:node --from=builder /app/build/index.js /app

EXPOSE 4500

ENTRYPOINT ["node", "index.js"]

Build a new image: docker build -t node-app .

We reduced the size by also three times. Start the container again and make it works.

Caveat: One of the drawbacks of bundling everything into a single file is that we don't have a human-readable stack trace of an error or a log message. To fix that, you can generate source maps of the file and package it in the Docker image, but this solution is not recommended as it slows the application.

In an upcoming post, I will show the recommended approach to fixing this issue.

Wrap up

When building a Docker image, take advantage of multi-stage builds, and use Esbuild to package your application in a single file when the size is critical (running on an AWS Lambda Function).

Use the tool Dive to explore the content of the Docker image.

You can find the code source on the GitHub repository.

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