Comprehensive Guide to Docker Images: Architecture, Principles, and Usage

Docker Images

Docker images are lightweight, standalone executable software packages that contain everything needed to run an application: code, runtime, system tools, libraries, etc.

As shown in the figure below:

Docker images primarily solve the following problems:

Environment isolation : Images provide the filesystem and runtime environment required by containers, achieving isolation between applications.

Environment consistency : Using Docker images ensures consistency across different environments, avoiding issues caused by environment differences.

Rapid deployment and scaling : Docker images enable fast deployment and scaling of applications, simplifying infrastructure management.

Portability : Docker images are lightweight and highly portable, allowing quick deployment and execution in various environments.

Docker Image Architecture

Docker images are the foundation for building Docker containers and can be viewed as the filesystem and configuration of a container.

As shown in the figure below:

Docker images serve as templates for creating and starting containers.

Multiple containers can be launched from the same image; they are independent but share the same image filesystem and configuration.

Docker images can be pulled from repositories, typically from Docker Hub.

As illustrated:

Docker Hub is a public image repository offering a large number of open‑source and official images, where developers can find images for operating systems, databases, web servers, development tools, and more.

Using the Docker CLI command docker pull , you can pull images from Docker Hub to your local environment and then create and run containers.

Docker Image Principles

Docker images are based on layered storage and UnionFS technology, constructing a complete filesystem view by stacking and merging layers.

1. Layered Storage

Docker images consist of multiple read‑only layers, each containing a set of filesystem changes.

These layers are stacked in order; each layer is read‑only and can be shared and reused.

2. Union File System

Docker uses a Union File System (UnionFS) to overlay these read‑only layers into a unified filesystem view.

UnionFS, implemented in Linux, allows multiple filesystems to be mounted at the same mount point, merging their contents.

The UnionFS lets users see a transparent filesystem view without needing to understand the underlying implementation.

In summary, Docker images are the foundation of Docker containers, acting as templates that, combined with containers, enable rapid deployment, isolated execution, and environment consistency.

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