01. What is Docker

What is Docker

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Prerequisites

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1. What is Docker

Docker is a containerization platform that allows developers to package an application and its dependencies into a lightweight, portable unit called a container.

Unlike virtual machines, containers share the host OS kernel, making them:

• Faster to start
• More lightweight
• Easier to deploy consistently

2. Key Components

Term	Description
Image	Blueprint for containers
Container	Running instance of an image
Dockerfile	Instructions to build an image
Volume	Persistent storage
Network	Communication between containers
Registry	Storage for images

2-1. Docker Engine

The core runtime that manages containers.

It consists of:

• Docker Daemon (dockerd) → Runs in the background (Back-end server)
• Docker CLI → User interface (docker run, docker build, etc.)
• REST API → Communication layer between CLI and daemon

[ You (CLI) ]
      ↓
docker command (CLI)
      ↓
REST API Request
      ↓
Docker Daemon (dockerd)
      ↓
Container / Image

Daemon:

• build image (docker build)
• run container (docker run)
• manage container (docker start, stop, delete)
• manage network and volumn

2-2. Docker Image

A read-only template used to create containers.

Includes:

• Application code
• Runtime (e.g., Python, Node.js)
• Libraries & dependencies

Built using a Dockerfile

2-3. Docker Container

A running instance of an image.

Characteristics:

• Isolated environment
• Has its own filesystem, network, and processes
• Can be started, stopped, deleted

2-4. Dockerfile

A script that defines how to build an image.

FROM ubuntu:24.04
RUN apt-get update && apt-get install -y python3
COPY . /app
CMD ["python3", "main.py"]

2-5. Docker Registry

A place to store and share images.

• Docker Hub (public)
• Private registries

3. How Docker works

3-1. Write Dockerfile

Defines environment and dependencies

3-2. Build Image

docker build -t myapp .

3-3. Store Image (Optional)

docker push myapp

3-4. Run Container

docker run myapp

4. Internal Working Principle

Docker relies on Linux kernel features:

4-1. Namespaces (Isolation)

Provide separation for:

• Process IDs
• Network
• Filesystem
• Users

Each container thinks it’s running alone

4.2 Cgroups (Resource Control)

Control resource usage:

• CPU
• Memory
• Disk I/O

4.3 Union File System (Layered FS)

Images are built in layers:

Base Image (Ubuntu)
   ↓
Install Packages
   ↓
Copy App Code

Benefits:

• Reuse layers
• Faster builds
• Smaller size

5. Container vs Virtual Machine

Feature	Container	VM
OS	Shared	Separate
Size	Small	Large
Boot Time	Seconds	Minutes
Performance	Near-native	Slower