Docker is an open-source platform that lets you package an application and all its dependencies into a single unit called a container. That container can run on any machine that has Docker installed — no "it works on my machine" problems.

• Consistency — The app behaves the same in development, staging, and production.
• Isolation — Each container runs independently without interfering with others.
• Portability — Build once, run anywhere (Linux, Windows, Mac, cloud).
• Speed — Containers start in seconds, unlike virtual machines.
• Efficiency — Containers share the host OS kernel, so they use far less memory and disk space than VMs.

In short: VMs virtualise the entire hardware stack. Docker virtualises only the application layer on top of a shared OS kernel — making it much lighter and faster.

VM Stack                    Docker Stack
┌──────────────────┐        ┌──────────────────┐
│   Application    │        │   Application    │
│   Guest OS       │        │   Dependencies   │
│   Hypervisor     │        │   Docker Engine  │
│   Host OS        │        │   Host OS        │
│   Hardware       │        │   Hardware       │
└──────────────────┘        └──────────────────┘

Docker containers always run Linux processes internally. How that is achieved depends on your OS.

• Docker runs natively — containers share the Linux host kernel directly.
• No virtualisation layer needed. Best performance.

• macOS has no Linux kernel, so Docker Desktop runs a lightweight Linux VM (using Apple Hypervisor Framework) behind the scenes.
• Your containers run inside that VM. Docker Desktop hides this complexity.

• Two modes available:
  • WSL 2 (recommended) — Docker Desktop uses Windows Subsystem for Linux 2, which runs a real Linux kernel inside Windows. Fast and lightweight.
  • Hyper-V — Docker runs a Linux VM using Hyper-V. Older approach, more overhead.

macOS / Windows
┌─────────────────────────────┐
│  Docker Desktop             │
│  ┌───────────────────────┐  │
│  │  Linux VM / WSL2      │  │
│  │  ┌─────────────────┐  │  │
│  │  │  Container      │  │  │
│  │  └─────────────────┘  │  │
│  └───────────────────────┘  │
└─────────────────────────────┘

# Update package index
sudo apt update

# Install Docker
sudo apt install -y docker.io

# Start and enable Docker
sudo systemctl start docker
sudo systemctl enable docker

# Run without sudo (optional)
sudo usermod -aG docker $USER

# Verify
docker --version

1. Download Docker Desktop from docker.com
2. Open the .dmg file and drag Docker to Applications
3. Launch Docker Desktop from Applications
4. Verify in terminal:

docker --version

1. Enable WSL 2 — open PowerShell as Administrator:

wsl --install

2. Download Docker Desktop from docker.com
3. Run the installer — make sure "Use WSL 2" is checked
4. Restart your PC
5. Verify in PowerShell:

docker --version

This project is a simple Node.js + Express app containerised with Docker.

l2-M-53/
├── index.js
├── package.json
└── Dockerfile

FROM node:20-alpine          # Use lightweight Node.js 20 base image
WORKDIR /usr/src/app         # Set working directory inside container
COPY package.json ./         # Copy dependency list
RUN npm install --production # Install dependencies
COPY . .                     # Copy all source files
EXPOSE 5000                  # Declare the port the app listens on
CMD ["node", "index.js"]     # Command to start the app

Step 1 — Build the image

docker build -t my-express-app .

• -t my-express-app gives the image a name (tag)
• . means use the Dockerfile in the current directory

Step 2 — Run a container from the image

docker run -p 5000:5000 my-express-app

• -p 5000:5000 maps port 5000 on your machine to port 5000 inside the container

Step 3 — Open in browser

http://localhost:5000

You should see: Hello World!

# List running containers
docker ps

# List all containers (including stopped)
docker ps -a

# List all images
docker images

# Stop a container
docker stop <container_id>

# Remove a container
docker rm <container_id>

# Remove an image
docker rmi my-express-app