Understanding Multi-Stage Dockerfile: A Guide for Efficient Containerization

Introduction

Docker has revolutionized the way applications are deployed by providing a containerized environment that ensures consistency across development, testing, and production. A key aspect of Docker is the Dockerfile, which is used to define and build custom container images based on specific requirements.
However, when dealing with large applications, traditional Dockerfiles can result in unnecessarily large images, consuming excessive storage and making deployments less efficient. This is where Multi-Stage Dockerfiles
come into play, helping to optimize the image size and improve security by only including what is necessary for the final deployment.

In this blog, we will explore Multi-Stage Dockerfiles, their benefits, and how they enhance the deployment process. We’ll also walk through an example using Maven and TomEE to build and deploy a Java-based web application.

What is a Multi-Stage Dockerfile?
A Multi-Stage Dockerfile is an advanced feature of Docker that allows multiple stages within a single Dockerfile. Each stage can use a different base image, and only the necessary artifacts from the previous stages are copied to the final image. This significantly reduces the final image size and enhances security by eliminating unnecessary dependencies.

Key Benefits of a Multi-Stage Dockerfile:

• Optimized Image Size: Reduces storage usage by only including essential artifacts.
• Improved Security: The final image is free from unnecessary build tools, reducing potential attack vectors.
• Simplified Workflow: Eliminates the need for separate Dockerfiles for building and running applications.
• Better Maintainability: Keeps Dockerfiles clean and easier to manage.

Multi-Stage Dockerfile Example: Java-Based Application
Below is the overview of the Multi-stage Dockerfile
Let's break down this Multi-Stage Dockerfile for a Java-based application that uses Maven for building and TomEE for deployment as a base image.

# Build Stage
FROM maven: latest AS build
WORKDIR /app
COPY . .
RUN mvn clean package

# Deployment Stage
FROM tomee:latest  # Includes Java and Tomcat (TomEE)
WORKDIR /app
COPY --from=build /app/target/*.war /usr/local/tomee/webapps/ROOT.war
EXPOSE 8080
CMD ["catalina.sh", "run"]

Stage 1: Build Stage

• Base Image: maven:latest – Includes Maven to compile the Java project.
• Working Directory: /app
• COPY Instruction: Copies all source code and the pom.xml file from the local machine to the container.
• RUN Instruction: Executes mvn clean package, which compiles the application and creates a .war file in the target directory.

Stage 2: Deployment Stage

• Base Image: tomee:latest – Includes Java and TomEE.
• Working Directory: /app
• COPY --from=build: Extracts only the .war file from the previous stage and moves it to TomEE's deployment directory (/usr/local/tomee/webapps/ROOT.war).
• EXPOSE 8080: Opens port 8080 to allow access to the application.
• CMD Instruction: Starts the TomEE server.

Why Use ROOT.war?
By default, a .war file deployed inside webapps/ is accessible via:

http://localhost:8080/<context_path>

However, when named ROOT.war, the application is deployed as the root, making it accessible directly at:

http://localhost:8080/

This eliminates the need to specify the context path in the URL.

Why Use a Multi-Stage Dockerfile?

A traditional approach would involve creating separate images for the build and deployment, but this can lead to large image sizes and unnecessary complexity. Here’s why using a Multi-Stage Dockerfile is beneficial:

1. Reduces Image Size: A standard image containing Maven and build dependencies can be several gigabytes in size. With a Multi-Stage Dockerfile, only the essential .war file is included in the final image, significantly reducing its size.
2. Enhances Security: By removing unnecessary build tools from the final image, we minimize the attack surface, making the containerized application more secure.
3. Improves Deployment Efficiency: Since the final image only contains runtime dependencies, it results in faster deployment times and efficient resource utilization.
4. Encourages Best Practices: Keeping build and runtime environments separate aligns with industry best practices, ensuring better maintainability and scalability.

Conclusion

Multi-Stage Dockerfiles are an essential feature for optimizing Docker images, reducing storage requirements, and improving security. By carefully structuring build and deployment stages, you can streamline the development workflow and deploy lightweight, efficient containers.

In this guide, we explored a practical Java-based example using Maven and TomEE, highlighting the benefits of this approach. Whether you're developing microservices, monolithic applications, or frontend-heavy web apps, adopting Multi-Stage Dockerfiles will greatly enhance your containerization strategy.

Happy Containerizing!