Docker Basics

What problem does Docker solve?

Imagine this scenario (which happens ALL the time):

Docker solves this by packaging your application WITH EVERYTHING it needs: code, runtime, libraries, configuration. If it works in Docker, it works anywhere.

🖥️ Virtual Machines vs Containers

    VIRTUAL MACHINE                    CONTAINER (DOCKER)
    ───────────────                    ──────────────────

    ┌─────────────────┐               ┌──────┐ ┌──────┐ ┌──────┐
    │      App        │               │ App1 │ │ App2 │ │ App3 │
    ├─────────────────┤               ├──────┴─┴──────┴─┴──────┤
    │  Libs/Binaries  │               │     Libs/Binaries      │
    ├─────────────────┤               ├────────────────────────┤
    │   Guest OS      │  ← Full OS    │     Docker Engine      │
    │  (Ubuntu 8GB)   │               │       (~100MB)         │
    ├─────────────────┤               ├────────────────────────┤
    │   Hypervisor    │               │        Host OS         │
    ├─────────────────┤               ├────────────────────────┤
    │    Host OS      │               │       Hardware         │
    ├─────────────────┤               └────────────────────────┘
    │   Hardware      │
    └─────────────────┘

    🐢 Heavy: 10-20 GB per VM         🚀 Light: 100-500 MB
    ⏱️ Boot: 1-2 minutes              ⏱️ Boot: 1-2 seconds
    📦 Full isolation                 📦 Process-level isolation

💰 Resource savings

💡 On a 32GB RAM server: you can run ~4 VMs or ~50 containers doing the same thing.

Why is it essential for the modern developer?

🔧 Docker alternatives

Docker popularized containers, but it's not the only option:

💡 Tip: If you use Linux and want to avoid Docker Desktop, Podman is the most popular alternative. Commands are almost identical: podman run instead of docker run.

Key concepts

Installation

# Verify installation
docker --version
docker run hello-world

Container lifecycle

    Dockerfile     docker build      Image         docker run      Container
    ──────────  ─────────────────▶  ────────  ───────────────────▶  ──────────
    (recipe)                        (frozen                         (served)
                                    dish)

    docker push                     docker pull
    ───────────▶ Docker Hub ◀───────────────────
                 (registry)

Your first container

# Test that Docker works
docker run hello-world

# Run Ubuntu interactively
docker run -it ubuntu bash

# Inside the container:
cat /etc/os-release  # Ubuntu!
exit                 # Exit

Essential commands

# See running containers
docker ps

# See all containers
docker ps -a

# See downloaded images
docker images

# Run with interactive shell
docker run -it ubuntu bash

# Stop container
docker stop <container_id>

# Remove container
docker rm <container_id>

Practical example: Node.js

# Run Node.js 20
docker run -it node:20 node

# Now you can write JavaScript
> console.log("Hello from Docker!")

Your first Dockerfile

FROM node:20
WORKDIR /app
COPY . .
RUN npm install
CMD ["npm", "start"]

# Build image
docker build -t my-app .

# Run
docker run -p 3000:3000 my-app

Practice

→ Docker Hello World — Your first container

Useful links

• 📖 Docker Docs
• 🎓 Docker Getting Started