Chapter 3. Docker Image and Container

Docker images and containers are core to containerized application development. They let developers create, deploy, and manage applications efficiently. In this chapter, you'll learn how Docker images act as blueprints for containers, the role of registries like Docker Hub, and how to handle the entire lifecycle of a container. We’ll cover both essential and advanced lifecycle management techniques and commands to help you work effectively with running containers. By the end, you'll have a strong foundation in managing Docker images and containers, empowering you to streamline your development process.

Understanding the Difference Between Docker Images and Containers

Although we’ll explore these two concepts in detail in this chapter, here's a brief overview of their key differences. Beginners often find Docker images and containers confusing because, although both are essential, they serve different roles. A Docker image is a static blueprint, while a Docker container is a live instance of that blueprint.

Docker Images: The Blueprint

A Docker image is a read-only template that includes the operating system, dependencies, configurations, and application code needed to create a container. Images are immutable—if you need to make changes, you must create a new version. They are portable and version-controlled (e.g., tagged as v1.0 or latest), ensuring consistency across development, testing, and production environments.

Docker Containers: The Running Instance

A Docker container is a live, executable instance of an image where applications run. Each container operates in an isolated environment with its own filesystem, processes, and network settings, which prevents conflicts between applications. Containers are modifiable—they can generate temporary files or store runtime data. However, without persistent storage, any data created is lost when the container stops or is removed. They are lightweight and ephemeral, designed to be quickly created and disposed of as needed.

Analogy: A Recipe vs. A Cooked Meal

• Docker Image: Think of it as a recipe with precise instructions, listing the ingredients (dependencies) and steps (configurations) required.
• Docker Container: This is like the actual meal prepared from that recipe. It is ready to use, can be modified, and is discarded when no longer needed.

Understanding these differences helps you build, manage, and deploy containerized applications more effectively.

What We Cover in This Chapter

In this chapter, we explain the fundamentals of Docker images and containers, focusing on their creation, management, and best practices. The following topics are covered:

Docker Images and Registries (Docker Hub)

Docker Hub is the central repository for sharing and distributing containerized applications. This section examines the role of Docker images, discusses public and private registries, and outlines best practices for managing images. You’ll work through a step-by-step case study on pulling and running an image from Docker Hub, and on tagging and pushing custom images to a registry. By understanding how registries work, you’ll efficiently store and distribute containerized applications.

Docker Container Lifecycle

Containers follow a structured lifecycle from creation to removal. This section explains the key stages of a container’s lifecycle and the commands needed to manage its state. You’ll complete a hands-on case study using an Nginx container, learning how to create, start, stop, restart, and remove containers. Grasping the container lifecycle is essential for effective Docker use.

Advanced Container Lifecycle Management

Docker offers advanced features for efficient container management. In this section, you’ll learn commands like docker restart, docker pause, and docker commit to optimize performance, reduce downtime, and automate management. We also cover best practices for cleaning up unused containers and managing persistent container state, ensuring your Docker environment remains organized.

Docker Commands to Interact Inside Containers

Interacting with running containers is key for debugging and managing applications. This section introduces commands such as docker exec and docker cp, which let you inspect processes, modify files, and transfer data between the host and containers. Through real-world case studies, you’ll practice modifying a live Nginx server and transferring files using docker cp, boosting your confidence in managing and troubleshooting containers.

By the end of this chapter, you'll clearly understand how Docker images and containers work. With practical experience managing container lifecycles, interacting with live containers, and using registries like Docker Hub, you’ll be well-equipped to streamline containerized application development.