Building Minimal Docker Images

In this exercise, you will focus on optimizing a Docker image by minimizing its size. Using the Dockerfile from the previous exercise, "Containerizing a Legacy Application," you will learn how to reduce the image size while ensuring that the application still runs correctly. This optimization is crucial for faster deployments, lower storage requirements, and improved performance.

The original Docker image created in the previous exercise is approximately 800 MB in size. In this exercise, your goal is to significantly reduce this size.

Requirements

Starting from the Dockerfile created in the previous exercise, you need to modify it to reduce the image size. The goal is to eliminate unnecessary layers and dependencies while maintaining the functionality of the Dockerized application.

To successfully complete this exercise, you need to:

• Refactor the Dockerfile to use multi-stage builds, separating the build environment from the runtime environment.
• Remove any unnecessary files or dependencies to minimize the final image size.
• Ensure that the final image only contains what is necessary to run the application.

Resources

• Docker Official Documentation
• Dockerfile Reference
• Best Practices for Writing Dockerfiles
• Project GitHub repository: https://github.com/tdevsin/docker-mastery.git

Acceptance Criteria

1. Dockerfile Refactored: The Dockerfile must be refactored to use multi-stage builds.
2. Image Size Reduced: The final Docker image size must be significantly reduced compared to the original Dockerfile. Expected value is less than 300 MB.
3. Application Runs: The application must run correctly inside the optimized Docker container.
4. Environment Variables Managed: Ensure the necessary environment variables are correctly passed and handled.

Hints to implement

• Use multi-stage builds to separate the build dependencies from the runtime environment.
• Consider using a smaller base image for the final stage, such as eclipse-temurin:21-jre, which includes only the JRE (Java Runtime Environment) instead of the full JDK (Java Development Kit). This will significantly reduce the image size. Research around smaller images.
• Test the image after each change to ensure the application still runs as expected.