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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.

Next Steps​

  1. Install Docker - Get Docker Desktop (Mac/Windows) or Docker Engine (Linux)
  2. Learn Docker Commands - Master essential CLI commands
  3. Create Dockerfiles - Build custom images
  4. Try Docker Compose - Manage multi-container apps
  5. Explore Docker Hub - Find pre-built images

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

What Can You Do with Docker?​

  • Web Development - Containerize Node.js, Python, PHP applications
  • Microservices - Build and deploy scalable service architectures
  • Development Environment - Consistent dev setups across teams
  • CI/CD Pipelines - Automated testing and deployment
  • Database Management - Run isolated database instances
  • Cloud Deployment - Deploy anywhere with container orchestration

Docker vs Virtual Machines​

FeatureDocker ContainersVirtual Machines
Resource UsageLightweight, shares OS kernelHeavy, full OS per VM
Startup TimeSecondsMinutes
IsolationProcess-levelHardware-level
PortabilityHighMedium
PerformanceNear-nativeOverhead from hypervisor

Good to Know​

  1. Docker containers share the host OS kernel, making them much more efficient than VMs
  2. Container images are built in layers, enabling efficient storage and transfer
  3. You can run multiple containers from the same image
  4. Containers are stateless by default - use volumes for persistent data
  5. Docker works on Windows, macOS, and Linux

Docker has revolutionized software development and deployment because:

  • "It Works on My Machine" Problem Solved Docker ensures your application runs the same way everywhere, eliminating environment-related bugs.

  • Faster Development Cycles Developers can quickly spin up consistent environments without complex setup procedures.

  • Efficient Resource Utilization Containers use fewer resources than virtual machines while providing similar isolation benefits.

  • Simplified Deployment Package your application with all dependencies into a single, portable container image.

  • Scalability Easily scale applications up or down by running multiple container instances.

  • DevOps Integration Perfect fit for CI/CD pipelines, enabling automated testing and deployment workflows.

Ready to dive deeper? Let's explore Docker installation and setup in the next section! 🚀