Understanding Docker Images, Containers, and Repositories: A Beginner’s Guide

Docker Image

Docker images (Image) are similar to virtual machine images and can be understood as read‑only templates for the Docker engine, containing a file system.

For example, an image may contain a complete Ubuntu operating system environment, called an Ubuntu image, or it may have Apache installed, referred to as an Apache image.

Users can create their own images or download ready‑made application images from the Internet and use them directly with simple commands.

Images are the foundation for creating Docker containers. Through versioning and an incremental file system, Docker provides a simple mechanism to create and update existing images.

Docker Container

Docker containers (Container) are lightweight sandboxes that Docker uses to run and isolate applications.

A container is an application runtime instance created from an image; it can be started, stopped, and deleted, and containers are isolated from each other.

A container can be seen as a simplified Linux environment—including root privileges, process space, user space, and network space—packaged together with the application it runs.

The image itself remains read‑only. When a container starts from an image, Docker creates a writable layer on top of the image while keeping the original image unchanged.

Docker Repository

Docker repositories (Repository) are similar to code repositories and serve as centralized locations for storing image files.

Each repository holds a specific class of images, often containing multiple image files distinguished by tags (e.g., an Ubuntu repository may include images for versions 14.04, 12.04, etc.).

Based on whether the stored images are publicly shared, Docker repositories can be public or private.

There is also the concept of a registry server, which hosts repositories and stores multiple repositories.