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Docker for Beginners: The Complete Step-by-Step Guide

Nov 6, 202516 min read

Master Docker with this complete beginner's guide. Learn containerization fundamentals, build real .NET applications, and understand why Docker is essential for modern software development—all explained with simple analogies and hands-on examples.

What is Docker?​

Docker is like a shipping container for your software. Just as shipping containers revolutionized global trade by providing a standard way to transport goods anywhere in the world, Docker revolutionizes software deployment by packaging your application and everything it needs into a standard container that runs anywhere.

At its core, Docker is an open-source platform that lets you package your application with all its dependencies, libraries, and configuration files into a single unit called a container. This container runs consistently on any computer, server, or cloud platform that has Docker installed.

The "It Works on My Machine" Problem​

Every developer has experienced this frustration:

  • Your app works perfectly on your laptop
  • Your teammate downloads the code and it crashes
  • QA reports bugs that you can't reproduce
  • Production breaks even though staging was fine

Why? Because each environment has different:

  • Operating system versions
  • Installed libraries and dependencies
  • Configuration settings
  • System tools and utilities

Docker solves this by bundling your app with its entire environment. Now "it works on my machine" means "it works on EVERY machine."

Real-World Analogy: The Restaurant Recipe​

Think of Docker like a complete restaurant recipe system:

  • Recipe (Dockerfile): Step-by-step instructions to prepare your dish
  • Prepared Meal Kit (Image): All ingredients pre-measured and packaged together
  • Served Dish (Container): The actual meal being eaten by customers

Just as a meal kit ensures anyone can cook the same dish with identical results, a Docker container ensures your app runs identically everywhere.

Why Should You Care About Docker?​

Docker isn't just a trendy tool—it solves real problems that developers face daily.

1. Consistency Across Environments​

The Problem: You develop on Windows. Your CI runs on Linux. Production is Ubuntu 22.04. Each environment behaves differently, causing deployment nightmares.

The Solution: One Docker container runs identically across all environments. What works in development WILL work in production.

2. Quick Setup for New Developers​

The Problem: New team members spend their first week installing databases, configuring environment variables, and hunting down version conflicts.

The Solution: Run docker compose up and have a complete development environment running in minutes, not days.

3. Resource Efficiency​

The Problem: Virtual machines are heavy. Each VM needs its own operating system, consuming gigabytes of memory and taking minutes to start.

The Solution: Docker containers share the host operating system kernel. They're lightweight (megabytes vs. gigabytes), start in milliseconds, and use minimal resources.

4. Microservices and Scaling​

The Problem: Modern apps consist of multiple services (API, database, cache, worker processes). Managing and scaling them is complex.

The Solution: Each service runs in its own container. Scale individual services independently based on demand.

Docker vs. Virtual Machines: What's the Difference?​

Understanding this difference is crucial for Docker beginners.

Virtual Machines (VMs)​

Think of a VM as owning an entire house:

  • You have your own structure, plumbing, and electrical system
  • Complete isolation but resource-heavy
  • Each VM runs a full operating system (Windows, Linux, etc.)
  • Typical size: 500MB to several GB
  • Boot time: 30 seconds to minutes

Docker Containers​

Think of a container as renting an apartment:

  • You share building infrastructure (elevators, water, power)
  • Still completely private and isolated
  • Shares the host OS kernel, only packages your app
  • Typical size: 10MB to 200MB
  • Boot time: Milliseconds

When to Use Each​

Use Virtual Machines:

  • Running different operating systems (Windows and Linux on same host)
  • Strict security isolation requirements
  • Legacy applications needing specific OS versions

Use Docker Containers:

  • Modern applications on Linux
  • Microservices architecture
  • Fast, reproducible development environments
  • Lightweight, scalable cloud deployments

Core Docker Concepts​

Let's break down the building blocks of Docker.

1. Docker Images: The Blueprint​

A Docker image is a read-only template that contains everything your app needs:

  • Application code
  • Runtime environment (.NET, Node.js, Python, etc.)
  • System libraries and tools
  • Configuration files
  • Environment variables

Think of an image as a snapshot or a blueprint. You can't modify it—if you need changes, you create a new image.

Key Characteristics:

  • Immutable: Once created, never changes
  • Layered: Built from stacked filesystem layers
  • Versioned: Tagged with versions (like myapp:1.0.0)
  • Portable: Works on any system with Docker
  • Reusable: One image can spawn multiple containers

2. Docker Containers: The Running Instance​

A container is a live, running instance of an image.

Using our recipe analogy:

  • Image = Recipe card
  • Container = The actual meal being cooked and served

Key Characteristics:

  • Ephemeral: Temporary and disposable
  • Isolated: Has its own filesystem, network, and processes
  • Lightweight: Shares the OS kernel with host
  • Multiple instances: You can run many containers from one image

Container Lifecycle:

3. Dockerfile: The Recipe​

A Dockerfile is a text file with instructions for building an image. It's your recipe.

Common Instructions:

  • FROM: Start from a base image (like Ubuntu or .NET)
  • WORKDIR: Set the working directory
  • COPY: Copy files from your computer into the image
  • RUN: Execute commands (install packages, build code)
  • ENV: Set environment variables
  • EXPOSE: Document which ports the app uses
  • CMD or ENTRYPOINT: Specify the command to run when container starts

Example Dockerfile:

# Start with .NET runtime
FROM mcr.microsoft.com/dotnet/runtime:8.0

# Set working directory
WORKDIR /app

# Copy application files
COPY bin/Release/net8.0/publish/ .

# Run the application
ENTRYPOINT ["dotnet", "MyApp.dll"]

4. Docker Volumes: Persistent Storage​

By default, when a container stops, all data inside it is lost. Volumes solve this problem by providing persistent storage.

Think of volumes as external hard drives:

  • Data survives when containers are deleted
  • Multiple containers can share the same volume
  • Perfect for databases, logs, and user uploads

Example:

# Create a named volume
docker volume create myapp-data

# Run container with volume
docker run -v myapp-data:/app/data myapp:latest

5. Docker Networks: Container Communication​

Networks allow containers to talk to each other.

By default, containers are isolated. Networks connect them:

  • Bridge Network: Default network for containers on same host
  • Custom Networks: Containers can find each other by name
  • Host Network: Container uses host's network directly

Example:

# Create network
docker network create myapp-network

# Run containers on the network
docker run --network myapp-network --name database postgres:15
docker run --network myapp-network --name api myapp:latest

# Now 'api' can connect to 'database' by name!

Getting Started: Installing Docker​

Let's get Docker running on your machine.

For Windows​

Requirements:

  • Windows 10/11 (64-bit) Pro, Enterprise, or Education
  • OR Windows 10/11 Home with WSL 2 enabled
  • At least 4GB RAM (8GB recommended)

Steps:

  1. Download Docker Desktop for Windows
  2. Run the installer
  3. Enable WSL 2 if prompted
  4. Restart your computer
  5. Launch Docker Desktop

For macOS​

Requirements:

  • macOS 11 (Big Sur) or newer
  • Apple Silicon or Intel chip
  • At least 4GB RAM (8GB recommended)

Steps:

  1. Download Docker Desktop for Mac
  2. Drag Docker.app to Applications folder
  3. Open Docker Desktop from Applications
  4. Follow the setup wizard

For Linux (Ubuntu/Debian)​

# Update package index
sudo apt-get update

# Install prerequisites
sudo apt-get install ca-certificates curl

# Add Docker's GPG key
sudo install -m 0755 -d /etc/apt/keyrings
sudo curl -fsSL https://download.docker.com/linux/ubuntu/gpg -o /etc/apt/keyrings/docker.asc
sudo chmod a+r /etc/apt/keyrings/docker.asc

# Add Docker repository
echo \
  "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/ubuntu \
  $(. /etc/os-release && echo "$VERSION_CODENAME") stable" | \
  sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

# Install Docker
sudo apt-get update
sudo apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin

Verify Installation​

Open your terminal and run:

docker --version
docker run hello-world

If you see "Hello from Docker!" you're ready to go!

What Just Happened?

  1. Docker looked for the hello-world image locally
  2. Didn't find it, so downloaded it from Docker Hub
  3. Created a container from the image
  4. Ran the container, which printed a message
  5. Container exited after finishing its task

Your First Docker Project: A .NET Console App​

Let's build a real application with Docker step by step.

Step 1: Create a Simple .NET Console Application​

Open your terminal and create a new .NET console app:

dotnet new console -o MyDockerApp
cd MyDockerApp

This creates a basic "Hello World" app. Let's make it more interesting. Open Program.cs and replace the code:

using System;
using System.Threading;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("🐳 Hello from Docker!");
        Console.WriteLine("============================");
        Console.WriteLine();
        
        Console.WriteLine("Environment Information:");
        Console.WriteLine($"Machine Name: {Environment.MachineName}");
        Console.WriteLine($"OS Version: {Environment.OSVersion}");
        Console.WriteLine($"Runtime: {Environment.Version}");
        Console.WriteLine($"Current Time: {DateTime.Now}");
        Console.WriteLine();
        
        Console.WriteLine("This app is running inside a Docker container!");
        Console.WriteLine("Press Ctrl+C to stop...");
        Console.WriteLine();
        
        // Keep app running
        int counter = 0;
        while (true)
        {
            counter++;
            Console.WriteLine($"[{DateTime.Now:HH:mm:ss}] Heartbeat #{counter}");
            Thread.Sleep(3000);  // Wait 3 seconds
        }
    }
}

Test it locally:

dotnet run

You should see output with heartbeats every 3 seconds. Press Ctrl+C to stop.

Step 2: Create a Dockerfile​

Create a file named Dockerfile (no extension) in your project folder:

# Build stage - uses SDK to compile the app
FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src

# Copy project file and restore dependencies
COPY *.csproj ./
RUN dotnet restore

# Copy all files and build
COPY . ./
RUN dotnet publish -c Release -o /app/publish

# Runtime stage - uses smaller runtime image
FROM mcr.microsoft.com/dotnet/runtime:8.0
WORKDIR /app

# Copy published output from build stage
COPY --from=build /app/publish .

# Run the application
ENTRYPOINT ["dotnet", "MyDockerApp.dll"]

Understanding the Dockerfile:

Stage 1 - Build:

  • Uses the full SDK (includes compiler and build tools)
  • Copies source code
  • Compiles and publishes the app

Stage 2 - Runtime:

  • Uses lightweight runtime image (no SDK, no compiler)
  • Only copies the compiled output
  • Much smaller final image size

This multi-stage build approach is a best practice. It keeps your final image small and secure by excluding build tools.

Step 3: Build the Docker Image​

Build your image with a name and tag:

docker build -t mydockerapp:1.0 .

Breaking down the command:

  • docker build: Build an image
  • -t mydockerapp:1.0: Tag it as "mydockerapp" version "1.0"
  • .: Use current directory (where Dockerfile is)

Watch as Docker executes each instruction in your Dockerfile.

Step 4: Run Your Container​

Start a container from your image:

docker run --name my-running-app mydockerapp:1.0

You'll see your app running inside the container! Notice:

  • The machine name is different (it's the container ID)
  • The OS is Linux (even if you're on Windows/Mac!)
  • Your heartbeat messages appear every 3 seconds

Press Ctrl+C to stop the container.

Step 5: Run in Detached Mode​

Run the container in the background (detached mode):

docker run -d --name my-background-app mydockerapp:1.0

The -d flag runs it in the background. You get back your terminal immediately.

View logs:

docker logs my-background-app

# Follow logs in real-time
docker logs -f my-background-app

Stop the container:

docker stop my-background-app

Remove the container:

docker rm my-background-app

Step 6: Essential Docker Commands​

Here are commands you'll use daily:

# List running containers
docker ps

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

# List images
docker images

# Remove an image
docker rmi mydockerapp:1.0

# Remove all stopped containers
docker container prune

# View container details
docker inspect my-running-app

# Execute a command in running container
docker exec -it my-running-app /bin/bash

# View resource usage
docker stats

Building a More Complex App: .NET API with Database​

Let's create a multi-container application using Docker Compose.

The Application Architecture​

We'll build:

  • A .NET Web API
  • A PostgreSQL database
  • Redis cache

All connected through a Docker network.

Step 1: Create the .NET Web API​

dotnet new webapi -o MyApiApp
cd MyApiApp

Step 2: Create the Dockerfile for the API​

Dockerfile:

FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src

COPY *.csproj ./
RUN dotnet restore

COPY . ./
RUN dotnet publish -c Release -o /app/publish

FROM mcr.microsoft.com/dotnet/aspnet:8.0
WORKDIR /app
COPY --from=build /app/publish .

EXPOSE 8080
ENV ASPNETCORE_URLS=http://+:8080

ENTRYPOINT ["dotnet", "MyApiApp.dll"]

Step 3: Create docker-compose.yml​

This file orchestrates multiple containers:

version: '3.8'

services:
  # PostgreSQL Database
  postgres:
    image: postgres:15-alpine
    container_name: myapp-postgres
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: mysecretpassword
      POSTGRES_DB: myappdb
    ports:
      - "5432:5432"
    volumes:
      - postgres-data:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 10s
      timeout: 5s
      retries: 5
    networks:
      - myapp-network

  # Redis Cache
  redis:
    image: redis:7-alpine
    container_name: myapp-redis
    ports:
      - "6379:6379"
    volumes:
      - redis-data:/data
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 10s
      timeout: 3s
      retries: 5
    networks:
      - myapp-network

  # .NET Web API
  api:
    build:
      context: .
      dockerfile: Dockerfile
    container_name: myapp-api
    depends_on:
      postgres:
        condition: service_healthy
      redis:
        condition: service_healthy
    environment:
      - ASPNETCORE_ENVIRONMENT=Development
      - ConnectionStrings__Database=Host=postgres;Database=myappdb;Username=postgres;Password=mysecretpassword
      - Redis__ConnectionString=redis:6379
    ports:
      - "8080:8080"
    networks:
      - myapp-network

volumes:
  postgres-data:
  redis-data:

networks:
  myapp-network:
    driver: bridge

Step 4: Start Everything with One Command​

docker compose up -d

This single command:

  1. Creates a custom network
  2. Starts PostgreSQL container and waits for it to be healthy
  3. Starts Redis container and waits for it to be healthy
  4. Builds and starts your API container
  5. Connects everything together

View logs:

# All services
docker compose logs -f

# Specific service
docker compose logs -f api

Stop everything:

docker compose down

Stop and remove volumes:

docker compose down -v

Step 5: Useful Docker Compose Commands​

# Start services
docker compose up

# Start in background
docker compose up -d

# Stop services
docker compose down

# View logs
docker compose logs

# Execute command in service
docker compose exec api bash

# Rebuild images
docker compose build

# List running services
docker compose ps

# Restart a service
docker compose restart api

Docker Best Practices for Beginners​

1. Keep Images Small​

Use minimal base images:

# Instead of this (700MB+)
FROM ubuntu:latest

# Use this (5-40MB)
FROM alpine:latest

# Or official minimal images
FROM mcr.microsoft.com/dotnet/runtime:8.0-alpine

Use multi-stage builds:

# Build stage has all tools (large)
FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
# ... build steps ...

# Runtime stage is minimal
FROM mcr.microsoft.com/dotnet/runtime:8.0
COPY --from=build /app/publish .

2. Optimize Layer Caching​

Order matters! Put rarely-changing instructions first:

# Good - dependencies cached separately from code
FROM mcr.microsoft.com/dotnet/sdk:8.0
WORKDIR /src

# This layer rarely changes
COPY *.csproj ./
RUN dotnet restore

# This layer changes often
COPY . ./
RUN dotnet publish -c Release -o /app

3. Never Store Secrets in Images​

# ❌ Bad - password in image
ENV DB_PASSWORD=mysecret

# ✅ Good - pass at runtime
docker run -e DB_PASSWORD=mysecret myapp:latest

# ✅ Better - use Docker secrets or environment files
docker run --env-file .env myapp:latest

4. Use .dockerignore​

Create a .dockerignore file to exclude unnecessary files:

node_modules/
bin/
obj/
.git/
.vs/
*.md
Dockerfile
.dockerignore

5. Tag Your Images Properly​

# Bad - no version
docker build -t myapp .

# Good - semantic versioning
docker build -t myapp:1.2.3 .
docker tag myapp:1.2.3 myapp:latest

6. Run as Non-Root User​

# Create and use non-root user
RUN addgroup -S appgroup && adduser -S appuser -G appgroup
USER appuser

Common Docker Troubleshooting​

Issue: Container Exits Immediately​

Check the logs:

docker logs <container-id>

Common causes:

  • Application crashes on startup
  • Missing environment variables
  • Wrong entry point command

Issue: Port Already in Use​

# Find what's using the port
netstat -ano | findstr :8080   # Windows
lsof -i :8080                   # Mac/Linux

# Use a different port
docker run -p 8081:8080 myapp:latest

Issue: Cannot Connect to Container​

Check if container is running:

docker ps

Verify port mapping:

docker port <container-name>

Check networks:

docker network inspect <network-name>

Issue: Out of Disk Space​

# Check disk usage
docker system df

# Clean up
docker system prune -a --volumes

Warning: This removes all stopped containers, unused images, and volumes!

Next Steps: Where to Go from Here​

Congratulations! You now understand Docker fundamentals. Here's your learning path:

Immediate Next Steps​

  1. Practice: Containerize your own applications
  2. Experiment: Try different base images and configurations
  3. Learn: Explore Docker Hub for pre-built images
  4. Build: Create a multi-container project with Docker Compose

Intermediate Topics​

  • Docker networking in depth: Custom networks, DNS resolution
  • Docker volumes: Bind mounts, named volumes, volume drivers
  • Docker security: Image scanning, rootless mode, secrets management
  • Docker in CI/CD: Automated builds and deployments
  • Performance optimization: Layer caching, build optimization

Advanced Topics​

  • Container orchestration: Kubernetes, Docker Swarm
  • Production deployments: Cloud platforms (AWS ECS, Azure Container Instances)
  • Monitoring and logging: Prometheus, ELK stack, container health
  • Multi-architecture builds: ARM and x86 images

Frequently Asked Questions​

Q: Is Docker free?
A: Yes, Docker is open source and free to use. Docker Desktop is free for personal use and small businesses.

Q: Can I run Windows apps in Docker?
A: Yes! Docker supports both Linux and Windows containers. However, Windows containers require Windows Server or Windows 10/11 with specific features enabled.

Q: How is Docker different from Kubernetes?
A: Docker runs individual containers. Kubernetes is an orchestration platform that manages hundreds or thousands of containers across multiple servers.

Q: Do I need to learn Linux to use Docker?
A: Basic Linux knowledge helps but isn't required. Docker Desktop provides a GUI, and most commands are straightforward.

Q: Can I develop inside a Docker container?
A: Yes! Many developers use Docker for development environments. VS Code has excellent Docker integration with the Remote - Containers extension.

Q: Is Docker secure?
A: Docker containers provide good isolation. For production, follow security best practices: run as non-root, scan images for vulnerabilities, use minimal base images, and keep Docker updated.

Q: How much does Docker slow down my application?
A: Minimal overhead—typically less than 5%. Containers share the host kernel, so they're much faster than virtual machines.

Helpful Resources​

Official Documentation:

Learning Platforms:

Community:

Summary​

Docker transforms how we build, ship, and run applications by:

  • Solving the "works on my machine" problem
  • Providing consistent environments from development to production
  • Enabling fast, efficient deployments
  • Supporting modern microservices architectures
  • Reducing infrastructure costs through efficient resource usage

The concepts we covered:

  • Images: Read-only templates (blueprints)
  • Containers: Running instances of images
  • Dockerfile: Instructions to build images
  • Volumes: Persistent data storage
  • Networks: Container communication
  • Docker Compose: Multi-container orchestration

Start small, experiment often, and don't be afraid to break things. Docker makes it easy to reset and try again. Every container you build strengthens your understanding.

Happy containerizing! 🐳

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