docker

Docker Containerization Skill

Summary

Docker provides containerization for packaging applications with their dependencies into isolated, portable units. Containers ensure consistency across development, testing, and production environments, eliminating "works on my machine" problems.

When to Use

• Local Development: Consistent dev environments across team members
• CI/CD Pipelines: Reproducible build and test environments
• Microservices: Isolated services with independent scaling
• Production Deployment: Portable applications across cloud providers
• Database/Service Testing: Ephemeral databases for integration tests
• Legacy Application Isolation: Run incompatible dependencies side-by-side

Quick Start

1. Create Dockerfile

FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

2. Build Image

docker build -t myapp:1.0 .

3. Run Container

docker run -p 3000:3000 myapp:1.0

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Core Concepts

Images vs Containers

• Image: Read-only template with application code, runtime, and dependencies
• Container: Running instance of an image with writable layer
• Registry: Storage for images (Docker Hub, GitHub Container Registry)

Layers and Caching

Each Dockerfile instruction creates a layer. Docker caches unchanged layers for faster builds.

# GOOD: Dependencies change less frequently than code
FROM python:3.11-slim
COPY requirements.txt .
RUN pip install -r requirements.txt  # Cached unless requirements.txt changes
COPY . .                              # Rebuild only when code changes

# BAD: Invalidates cache on every code change
FROM python:3.11-slim
COPY . .                              # Changes frequently
RUN pip install -r requirements.txt  # Reinstalls on every build

Volumes

Persistent data storage that survives container restarts.

# Named volume (managed by Docker)
docker run -v mydata:/app/data myapp

# Bind mount (host directory)
docker run -v $(pwd)/data:/app/data myapp

# Anonymous volume (temporary)
docker run -v /app/data myapp