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What is Docker?

Overview

Docker is a platform for building, packaging, and running applications in containers.

A container bundles:

  • Application code
  • Runtime
  • Dependencies
  • System libraries

This allows the application to run consistently across environments.

Why Docker Exists

Before containers, applications often failed due to environment differences:

"It works on my machine"

Docker solves this by:

  • Standardising environments
  • Isolating applications
  • Making deployments reproducible

What is a Container?

A container is:

  • A lightweight, isolated process
  • Running on a shared host OS kernel
  • Packaged with everything it needs to run

Unlike virtual machines, containers do not include a full operating system.

Docker Architecture

graph TD Developer --> DockerCLI DockerCLI --> DockerDaemon DockerDaemon --> Images DockerDaemon --> Containers DockerDaemon --> Registry Registry -->|Pull/Push| DockerDaemon

Key Components

Docker CLI

Command-line interface used by developers.

Docker Daemon

Background service that builds and runs containers.

Docker Images

Immutable templates used to create containers.

Docker Containers

Running instances of images.

Docker Registry

Stores and distributes images (e.g., Docker Hub).

How Docker Works

  1. Developer writes a Dockerfile
  2. Docker builds an image
  3. Image is stored locally or in a registry
  4. Container is started from the image

Real-World Implications

  • Containers start in seconds (vs minutes for VMs)
  • Applications become portable across environments
  • Infrastructure becomes easier to automate

Key Takeaways

  • Docker packages applications with dependencies
  • Containers are lightweight and fast
  • Images are immutable; containers are runtime instances