How Docker Really Works: Inside Images, Containers, and Registries

Docker is a container platform that packages applications and all dependencies into portable containers.

It enables the same container to run on any Docker‑compatible environment, solving the “works on my machine” problem.

Core Goal

“Build once, run anywhere”.

Docker Runtime Components

Docker’s runtime consists of three main components: images, containers, and registries.

Docker Image

An image is a read‑only template used to create containers. It contains all files and configuration needed to run an application and is built as a layered filesystem snapshot.

Each layer is created by a Dockerfile instruction (e.g., RUN, COPY) and stacked using a union filesystem such as OverlayFS.

Docker Container

A container is a writable runtime instance of an image, providing an isolated execution environment.

Linux namespaces isolate resources such as PID, network, mount points, IPC, UTS, and user IDs, giving each container the illusion of its own system.

PID – process isolation

NET – network isolation

MNT – mount point isolation

IPC – inter‑process communication isolation

UTS – hostname isolation

USER – user privilege isolation

Docker Registry

A registry stores and distributes images. Users can pull images to run containers locally or push locally built images to share them.

Typical Workflow

Write a Dockerfile to build an image, push the image to a registry (e.g., Docker Hub), then pull the image on a production host and start a container that runs the business application.

By tightly integrating images, containers, and registries, Docker enables rapid building, distribution, and execution of applications.