Chapter 5. Building and Sharing Docker Images

Docker images are an essential part of containerized applications, providing a lightweight, portable, and consistent runtime environment. In previous chapters, we covered how to run containers using pre-built images from Docker Hub, but what if you need a customized setup for your application? That’s where building and managing your own Docker images becomes important.

In this chapter, we’ll explore how to create images using Dockerfiles, optimize builds with a .dockerignore file, and share images across teams and repositories. Understanding these concepts is crucial for maintaining efficiency, consistency, and security in your development and deployment workflow.

So far, we've worked with Docker Official Images—trusted, well-maintained base images provided by Docker and its community. These images, such as those used in previous chapters, offered a ready-to-use environment for running applications. However, we haven’t yet explored how to create custom images tailored to specific project requirements or how to share them through a repository. This chapter bridges that gap, ensuring you have the necessary skills to build and distribute your own Docker images effectively.

Docker Official Images vs. Custom Images

Before building your own Docker images, it’s important to understand the different types of images available. In Docker, an image is a packaged environment that contains everything an application needs to run, including its operating system, libraries, dependencies, and configurations.

What Are Docker Official Images?

Docker Official Images are pre-built, publicly available images maintained by Docker and its open-source community. These images follow best practices, receive security updates, and are verified for reliability. Common examples include nginx, mysql, and node, which provide ready-to-use environments for running web servers, databases, and applications.

Throughout previous chapters, we have worked with official images to pull and run containers effortlessly. Since these images are reviewed and maintained by trusted contributors, they serve as a solid foundation for many projects.

What Are Custom Images?

While official images provide a great starting point, they may not always include everything your application requires. Custom images are Docker images that you build yourself to suit specific needs. They allow you to:

• Add required software and dependencies
• Configure application settings
• Optimize performance for your use case
• Ensure consistency across different environments

So far, we have only used pre-existing images from Docker Hub, but we haven’t covered how to create and customize images from scratch. This chapter will introduce you to that process and show you how to share your own images through a repository.

When to Use Each?

• Use Docker Official Images when you need a stable, secure, and well-tested foundation for your application. These images are ideal for standard use cases where reliability is key. We’ve already covered how to pull and use official images in previous chapters.
• Use Custom Images when your project requires additional software, specific configurations, or optimizations beyond what an official image provides. Building custom images gives you complete control over the application environment.

By understanding when to use each type, you can choose the best approach for your project. Whether you’re working with a pre-built official image or crafting a custom one from scratch, each decision brings you closer to mastering Docker containerization.

What We Cover in This Chapter

In this chapter, we explore the process of building and sharing Docker images—a crucial step in containerized application development. The following topics are covered:

What Is a Dockerfile?

A Dockerfile is a text file that contains step-by-step instructions for building a Docker image. It acts as a blueprint, ensuring consistency and automation in the containerization process. Instead of manually setting up application environments, developers define configurations, dependencies, and commands in a Dockerfile. This section covers the fundamentals of writing Dockerfiles, including efficient structuring. You will also learn the difference between running a container manually and using a Dockerfile, highlighting how automation improves scalability, repeatability, and collaboration.

Build Context and .dockerignore File

Understanding the build context is key to creating efficient Docker images. The build context includes all files and directories available to the Docker daemon during the image-building process. However, sending unnecessary files can slow down builds and introduce security risks. To prevent this, developers use a .dockerignore file to exclude irrelevant or sensitive data, ensuring that only essential files are included in the build. This section explains how the build context works, how to properly configure a .dockerignore file, and how these practices help optimize performance and security in Docker image creation.

Dockerfile Syntax

Writing an optimized Dockerfile requires an understanding of its syntax and core instructions. Dockerfiles consist of layered commands, with each instruction contributing to the final image structure. This section explores key Dockerfile instructions such as FROM, COPY, RUN, CMD, and ENTRYPOINT. You will also learn best practices for reducing image size, improving performance, and maintaining clean, readable instructions. By following structured syntax rules and avoiding common mistakes, you can create lightweight and efficient images tailored to your application’s needs.

Sharing Your Docker Images

Once a Docker image is built, sharing it with others is essential for collaboration and deployment. Docker Hub and other container registries provide platforms for distributing images publicly or privately. This section walks you through the image-sharing workflow, including tagging images, logging into a registry, pushing images, and managing access permissions. You will also learn how to automate image distribution for continuous integration and deployment (CI/CD) pipelines. By understanding how to efficiently store and share Docker images, you can streamline team collaboration and ensure smooth deployments across various environments.

By the end of this chapter, you'll be able to build and share Docker images with confidence. You'll understand how to create efficient Dockerfiles, optimize your build context, and share images effectively for smooth collaboration and deployment.