Docker Multi-Stage Builds

Last updated on November 29, 2023

Multi-stage builds in Docker allow you to create a Dockerfile with multiple build stages, enabling you to build and optimize your final image more efficiently. It’s particularly useful for reducing the size of the final image by separating the build environment from the runtime environment.

Here’s an example of a Dockerfile utilizing multi-stage builds for a Node.js application using Express.js:

# Stage 1: Build stage
FROM node:latest as builder

WORKDIR /app

# Copy package.json and package-lock.json to install dependencies
COPY package*.json ./
RUN npm install

# Copy application code
COPY . .

# Build the app (if needed)
# Replace this command with your build process if applicable
RUN npm run build

# Stage 2: Runtime stage
FROM node:slim

WORKDIR /app

# Copy only necessary files from the builder stage to the runtime stage
COPY --from=builder /app/package*.json ./
COPY --from=builder /app/node_modules ./node_modules
COPY --from=builder /app/.env ./
COPY --from=builder /app/dist ./dist   # Assuming compiled code is in the 'dist' directory

# Expose port if needed
EXPOSE 3000

# Command to start the application
CMD ["node", "dist/app.js"]

• Stage 1 (builder):
  • Uses the node:latest image as the base image for the build stage.
  • Sets the working directory to /app.
  • Copies package.json and package-lock.json to install dependencies and runs npm install.
  • Copies the application code (assuming the code is in the same directory as the Dockerfile).
  • Runs any build commands needed for your application (e.g., transpilation, bundling).
• Stage 2 (Runtime):
  • Uses a lighter Node.js image (node:slim) as the base image for the runtime stage.
  • Sets the working directory to /app.
  • Copies only necessary artifacts from the builder stage:
    • package*.json: Copying the package files to ensure the runtime dependencies are available.
    • node_modules: Copying the installed dependencies from the builder stage.
    • .env or other configuration files.
    • Compiled code or necessary application files (dist in this example).
  • Exposes the necessary port(s) if the application requires external access.
  • Specifies the command (CMD) to start the application. Adjust this according to your project’s file structure and entry point.

This Dockerfile separates the build and runtime environments, ensuring that the final image contains only the necessary artifacts and dependencies required to run the application. Adjust the CMD line to point to your actual application entry point if it’s different from dist/app.js.

As you read above Multi-Stage Builds come with several advantages and a few considerations to keep in mind:

Advantages:

1. Reduced Image Size: Multi-stage builds allow you to build intermediate images in separate stages and copy only necessary artifacts to the final image. This significantly reduces the size of the final Docker image.
2. Improved Performance: By separating build-time dependencies from runtime dependencies, the final image includes only the essential components needed to run the application, resulting in better performance.
3. Simplified Builds: Developers can manage complex build processes more easily within a single Dockerfile. This streamlines the build process and simplifies CI/CD pipelines.
4. Enhanced Security: Because the final image contains only what’s necessary for runtime, the attack surface and exposure to vulnerabilities are reduced. Unnecessary tools or dependencies present during the build phase aren’t included in the final image.
5. Better Maintainability: Multi-stage builds help maintain a cleaner Dockerfile and reduce clutter by segregating build steps, making it easier to understand and maintain the build process.

Considerations:

1. Complexity: Understanding and managing multi-stage builds might be challenging for beginners or those unfamiliar with Docker. Managing multiple stages and dependencies can add complexity.
2. Debugging Challenges: Troubleshooting issues in multi-stage builds, especially when dealing with multiple intermediate images, might be more complicated compared to traditional single-stage builds.
3. Learning Curve: Adopting multi-stage builds may require a learning curve for teams not familiar with Docker’s multi-stage functionality, potentially impacting productivity initially.
4. Build Caching Limitations: While multi-stage builds leverage build caching efficiently, certain steps might invalidate the cache, leading to longer build times in some scenarios.
5. Build Tool Dependency: Multi-stage builds might depend on specific build tools or configurations, and ensuring these tools are available and consistent across different environments can be a consideration.

Despite these considerations, the advantages of reduced image size, improved performance, and enhanced security often outweigh the challenges for many users, especially in large-scale or production environments. Understanding the trade-offs and using best practices can help maximize the benefits of Docker’s multi-stage builds while mitigating potential drawbacks.