As developers, we're always looking for ways to streamline our workflow and boost productivity. One of the most powerful tools in our arsenal is hot reloading - the ability to see our changes reflected in real-time without manually restarting our application. But what happens when we throw Docker into the mix? In this post, we'll walk through the process of enabling hot reloading for a Next.js application running inside a Docker container.

Why Docker?

Docker has become an essential tool in modern web development. It allows us to create consistent development environments, simplify deployment processes, and ensure that our application runs the same way across different machines. However, running a Next.js application in Docker can sometimes feel like we're sacrificing the rapid feedback loop we're used to in local development.

The Challenge

By default, when you run a Next.js application in a Docker container, changes to your local files aren't automatically reflected in the running application. This is because the container is isolated from your local file system. To bridge this gap and enable hot reloading, we need to make some specific configurations.

The Solution

To enable hot reloading for Next.js in Docker, we need to make changes to three key files: the Dockerfile, the Docker Compose file, and the Next.js configuration file. Let's break down each of these changes.

1. Updating the Dockerfile

First, we need to modify our Dockerfile to run Next.js in development mode:

FROM node:18-alpine

WORKDIR /app

COPY package.json package-lock.json ./
RUN npm ci

COPY . .
COPY next.config.mjs ./next.config.mjs

EXPOSE 3000

CMD ["npm", "run", "dev"]

The key changes here are:

• We're not running npm run build anymore.
• We've changed the CMD to npm run dev to start Next.js in development mode.

2. Modifying the Docker Compose File

Next, we need to update our Docker Compose file to enable volume mounting:

version: '3'
services:
  # ... other services ...

  frontend:
    build:
      context: ./frontend
      dockerfile: Dockerfile
    ports:
      - "3000:3000"
    environment:
      - BACKEND_URL=http://backend:8000
    volumes:
      - ./frontend:/app
      - /app/node_modules
    depends_on:
      - backend

The crucial additions here are:

• We've added volumes to mount the local frontend directory to /app in the container.
• We've included a volume for node_modules to prevent it from being overwritten by the local mount.

3. Adjusting the Next.js Configuration

Finally, we need to modify our Next.js configuration to enable file watching in Docker:

/** @type {import('next').NextConfig} */
const nextConfig = {
  // ... other configurations ...

  webpackDevMiddleware: config => {
    config.watchOptions = {
      poll: 1000,
      aggregateTimeout: 300,
    }
    return config
  },
}

export default nextConfig;

The added webpackDevMiddleware configuration enables polling for file changes, which is necessary for hot reloading to work in Docker.

Putting It All Together

With these changes in place, you can now run your Docker setup with hot reloading enabled. Use the following command to start your containers:

docker-compose up --build

Now, when you make changes to your Next.js application code, you should see those changes reflected almost immediately in your browser, just as if you were running Next.js directly on your host machine.

Conclusion

Enabling hot reloading for Next.js in Docker might seem like a small change, but it can significantly improve your development experience. It combines the consistency and isolation of Docker with the rapid feedback loop we've come to expect in modern web development.

Remember, the first time you run this setup, it might take a while to install all the dependencies. Subsequent runs should be much faster.

By following this guide, you've supercharged your Next.js development environment in Docker. Happy coding!