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🐳 Introduction to Docker

Welcome! Docker helps you package and run applications in containers—think of them as lightweight, portable boxes that contain everything your app needs to run.

What is Docker?​

Docker is a platform that lets you build, ship, and run applications inside containers. Instead of worrying about "it works on my machine" problems, Docker ensures your app runs the same way everywhere.

Simple analogy: Just like shipping containers standardized global trade, Docker containers standardize software deployment. Your application + its dependencies = one portable package.

Why Use Docker?​

  • Consistency - Same behavior on your laptop, your teammate's computer, and production servers
  • Fast setup - New developers can start working in minutes instead of days
  • Isolation - Each app runs in its own environment without conflicts
  • Efficiency - Containers are lightweight and start in seconds
  • Portability - Build once, run anywhere

Core Concepts​

Image​

A blueprint for your application. Contains your code, runtime, libraries, and configuration. Images are built from a Dockerfile and never change once created.

Container​

A running instance of an image. Lightweight, isolated, and disposable. You can run multiple containers from the same image.

Dockerfile​

A simple text file with instructions to build an image:

FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
CMD ["node", "app.js"]

Registry​

A storage service for Docker images. Docker Hub is the most popular—like GitHub for Docker images.

Volume​

Persistent storage that survives when containers are deleted. Use for databases, logs, and user files.

Network​

Allows containers to communicate with each other securely.

Quick Start Workflow​

1. Create a Dockerfile

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
CMD ["python", "app.py"]

2. Build your image

docker build -t my-app:1.0 .

3. Run a container

docker run -d --name my-app -p 8000:8000 my-app:1.0

4. Check it's running

docker ps
docker logs my-app

That's it! Your app is now running in a container.

Essential Commands​

# Build an image
docker build -t myapp:1.0 .

# Run a container
docker run -d --name myapp -p 8080:80 myapp:1.0

# List running containers
docker ps

# View logs
docker logs myapp

# Stop a container
docker stop myapp

# Remove a container
docker rm myapp

# List images
docker images

# Remove an image
docker rmi myapp:1.0

Best Practices​

✅ Use official base images - node:18-alpine, python:3.11-slim
✅ Use specific tags - Avoid :latest in production
✅ Keep images small - Use Alpine or slim variants
✅ Add .dockerignore - Exclude node_modules, .git, logs
✅ Don't run as root - Create a non-privileged user
✅ Use volumes for data - Never store important data in containers
✅ One process per container - Keep it simple and focused

Next Steps​

  1. Install Docker - Get Docker Desktop (Mac/Windows) or Docker Engine (Linux)
  2. Try the examples - Build and run the sample Dockerfiles above
  3. Learn Docker Compose - Manage multi-container apps easily
  4. Explore Docker Hub - Find pre-built images for databases, web servers, etc.
  5. Read the docs - https://docs.docker.com/

Key Takeaways​

  • Containers package your app with everything it needs
  • Images are blueprints, containers are running instances
  • Dockerfiles define how to build images
  • Docker Compose manages multiple containers together
  • Docker makes development, testing, and deployment much easier

Ready to containerize your first app? Start with a simple Dockerfile and experiment! 🚀