🚨 CRITICAL GUIDELINES

Windows File Path Requirements

MANDATORY: Always Use Backslashes on Windows for File Paths

When using Edit or Write tools on Windows, you MUST use backslashes (\) in file paths, NOT forward slashes (/).

Examples:

• ❌ WRONG: D:/repos/project/file.tsx
• ✅ CORRECT: D:\repos\project\file.tsx

This applies to:

• Edit tool file_path parameter
• Write tool file_path parameter
• All file operations on Windows systems

Documentation Guidelines

NEVER create new documentation files unless explicitly requested by the user.

• Priority: Update existing README.md files rather than creating new documentation
• Repository cleanliness: Keep repository root clean - only README.md unless user requests otherwise
• Style: Documentation should be concise, direct, and professional - avoid AI-generated tone
• User preference: Only create additional .md files when user specifically asks for documentation

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Docker Best Practices

This skill provides current Docker best practices across all aspects of container development, deployment, and operation.

Image Best Practices

Base Image Selection

2025 Recommended Hierarchy:

1. Wolfi/Chainguard (cgr.dev/chainguard/*) - Zero-CVE goal, SBOM included
2. Alpine (alpine:3.19) - ~7MB, minimal attack surface
3. Distroless (gcr.io/distroless/*) - ~2MB, no shell
4. Slim variants (node:20-slim) - ~70MB, balanced

Key rules:

• Always specify exact version tags: node:20.11.0-alpine3.19
• Never use latest (unpredictable, breaks reproducibility)
• Use official images from trusted registries
• Match base image to actual needs

Dockerfile Structure

Optimal layer ordering (least to most frequently changing):

1. Base image and system dependencies
2. Application dependencies (package.json, requirements.txt, etc.)
3. Application code
4. Configuration and metadata

Rationale: Docker caches layers. If code changes but dependencies don't, cached dependency layers are reused, speeding up builds.

Example:

FROM python:3.12-slim

# 1. System packages (rarely change)
RUN apt-get update && apt-get install -y --no-install-recommends \
    gcc \
    && rm -rf /var/lib/apt/lists/*

# 2. Dependencies (change occasionally)
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# 3. Application code (changes frequently)
COPY . /app
WORKDIR /app

CMD ["python", "app.py"]

Multi-Stage Builds

Use multi-stage builds to separate build dependencies from runtime:

# Build stage
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Production stage
FROM node:20-alpine AS runtime
WORKDIR /app
# Only copy what's needed for runtime
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
USER node
CMD ["node", "dist/server.js"]

Benefits:

• Smaller final images (no build tools)
• Better security (fewer attack vectors)
• Faster deployment (smaller upload/download)

Layer Optimization

Combine commands to reduce layers and image size:

# Bad - 3 layers, cleanup doesn't reduce size
RUN apt-get update
RUN apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

# Good - 1 layer, cleanup effective
RUN apt-get update && \
    apt-get install -y --no-install-recommends curl && \
    rm -rf /var/lib/apt/lists/*

.dockerignore

Always create .dockerignore to exclude unnecessary files:

# Version control
.git
.gitignore

# Dependencies
node_modules
__pycache__
*.pyc

# IDE
.vscode
.idea

# OS
.DS_Store
Thumbs.db

# Logs
*.log
logs/

# Testing
coverage/
.nyc_output
*.test.js

# Documentation
README.md
docs/

# Environment
.env
.env.local
*.local

Container Runtime Best Practices

Security

docker run \
  # Run as non-root
  --user 1000:1000 \
  # Drop all capabilities, add only needed ones
  --cap-drop=ALL \
  --cap-add=NET_BIND_SERVICE \
  # Read-only filesystem
  --read-only \
  # Temporary writable filesystems
  --tmpfs /tmp:noexec,nosuid \
  # No new privileges
  --security-opt="no-new-privileges:true" \
  # Resource limits
  --memory="512m" \
  --cpus="1.0" \
  my-image

Resource Management

Always set resource limits in production:

# docker-compose.yml
services:
  app:
    deploy:
      resources:
        limits:
          cpus: '2.0'
          memory: 1G
        reservations:
          cpus: '1.0'
          memory: 512M

Health Checks

Implement health checks for all long-running containers:

HEALTHCHECK --interval=30s --timeout=3s --retries=3 --start-period=40s \
  CMD curl -f http://localhost:3000/health || exit 1

Or in compose:

services:
  app:
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost/health"]
      interval: 30s
      timeout: 3s
      retries: 3
      start_period: 40s

Logging

Configure proper logging to prevent disk fill-up:

services:
  app:
    logging:
      driver: "json-file"
      options:
        max-size: "10m"
        max-file: "3"

Or system-wide in /etc/docker/daemon.json:

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}

Restart Policies

services:
  app:
    # For development
    restart: "no"

    # For production
    restart: unless-stopped

    # Or with fine-grained control (Swarm mode)
    deploy:
      restart_policy:
        condition: on-failure
        delay: 5s
        max_attempts: 3
        window: 120s

Docker Compose Best Practices

File Structure

# No version field needed (Compose v2.40.3+)

services:
  # Service definitions
  web:
    # ...
  api:
    # ...
  database:
    # ...

networks:
  # Custom networks (preferred)
  frontend:
  backend:
    internal: true

volumes:
  # Named volumes (preferred for persistence)
  db-data:
  app-data:

configs:
  # Configuration files (Swarm mode)
  app-config:
    file: ./config/app.conf

secrets:
  # Secrets (Swarm mode)
  db-password:
    file: ./secrets/db_pass.txt

Network Isolation

networks:
  frontend:
    driver: bridge
  backend:
    driver: bridge
    internal: true  # No external access

services:
  web:
    networks:
      - frontend

  api:
    networks:
      - frontend
      - backend

  database:
    networks:
      - backend  # Not accessible from frontend

Environment Variables

services:
  app:
    # Load from file (preferred for non-secrets)
    env_file:
      - .env

    # Inline for service-specific vars
    environment:
      - NODE_ENV=production
      - LOG_LEVEL=info

    # For Swarm mode secrets
    secrets:
      - db_password

Important:

• Add .env to .gitignore
• Provide .env.example as template
• Never commit secrets to version control

Dependency Management

services:
  api:
    depends_on:
      database:
        condition: service_healthy  # Wait for health check
      redis:
        condition: service_started   # Just wait for start

Production Best Practices

Image Tagging Strategy

# Use semantic versioning
my-app:1.2.3
my-app:1.2
my-app:1
my-app:latest

# Include git commit for traceability
my-app:1.2.3-abc123f

# Environment tags
my-app:1.2.3-production
my-app:1.2.3-staging

Secrets Management

Never do this:

# BAD - secret in layer history
ENV API_KEY=secret123
RUN echo "password" > /app/config

Do this:

# Use Docker secrets (Swarm) or external secret management
docker secret create db_password ./password.txt

# Or mount secrets at runtime
docker run -v /secure/secrets:/run/secrets:ro my-app

# Or use environment files (not in image)
docker run --env-file /secure/.env my-app

Monitoring & Observability

services:
  app:
    # Health checks
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost/health"]
      interval: 30s

    # Labels for monitoring tools
    labels:
      - "prometheus.io/scrape=true"
      - "prometheus.io/port=9090"
      - "com.company.team=backend"
      - "com.company.version=1.2.3"

    # Logging
    logging:
      driver: "json-file"
      options:
        max-size: "10m"
        max-file: "3"

Backup Strategy

# Backup named volume
docker run --rm \
  -v VOLUME_NAME:/data \
  -v $(pwd):/backup \
  alpine tar czf /backup/backup-$(date +%Y%m%d).tar.gz -C /data .

# Restore volume
docker run --rm \
  -v VOLUME_NAME:/data \
  -v $(pwd):/backup \
  alpine tar xzf /backup/backup.tar.gz -C /data

Update Strategy

services:
  app:
    # For Swarm mode - rolling updates
    deploy:
      replicas: 3
      update_config:
        parallelism: 1        # Update 1 at a time
        delay: 10s            # Wait 10s between updates
        failure_action: rollback
        monitor: 60s
      rollback_config:
        parallelism: 1
        delay: 5s

Platform-Specific Best Practices

Linux

• Use user namespace remapping for added security
• Leverage native performance advantages
• Use Alpine for smallest images
• Configure SELinux/AppArmor profiles
• Use systemd for Docker daemon management

// /etc/docker/daemon.json
{
  "userns-remap": "default",
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  },
  "storage-driver": "overlay2",
  "live-restore": true
}

macOS

• Allocate sufficient resources in Docker Desktop
• Use :delegated or :cached for bind mounts
• Consider multi-platform builds for ARM (M1/M2)
• Limit file sharing to necessary directories

# Better volume performance on macOS
volumes:
  - ./src:/app/src:delegated  # Host writes are delayed
  - ./build:/app/build:cached  # Container writes are cached

Windows

• Choose container type: Windows or Linux
• Use forward slashes in paths
• Ensure drives are shared in Docker Desktop
• Be aware of line ending differences (CRLF vs LF)
• Consider WSL2 backend for better performance

# Windows-compatible paths
volumes:
  - C:/Users/name/app:/app  # Forward slashes work
  # or
  - C:\Users\name\app:/app  # Backslashes need escaping in YAML

Performance Best Practices

Build Performance

# Use BuildKit (faster, better caching)
export DOCKER_BUILDKIT=1

# Use cache mounts
RUN --mount=type=cache,target=/root/.cache/pip \
    pip install -r requirements.txt

# Use bind mounts for dependencies
RUN --mount=type=bind,source=package.json,target=package.json \
    --mount=type=bind,source=package-lock.json,target=package-lock.json \
    --mount=type=cache,target=/root/.npm \
    npm ci

Image Size

• Use multi-stage builds
• Choose minimal base images
• Clean up in the same layer
• Use .dockerignore
• Remove build dependencies

# Install and cleanup in one layer
RUN apt-get update && \
    apt-get install -y --no-install-recommends \
    package1 \
    package2 && \
    apt-get clean && \
    rm -rf /var/lib/apt/lists/*

Runtime Performance

# Use exec form (no shell overhead)
CMD ["node", "server.js"]  # Good
# vs
CMD node server.js         # Bad - spawns shell

# Optimize signals
STOPSIGNAL SIGTERM

# Run as non-root (slightly faster, much more secure)
USER appuser

Security Best Practices Summary

Image Security:

• Use official, minimal base images
• Scan for vulnerabilities (Docker Scout, Trivy)
• Don't include secrets in layers
• Run as non-root user
• Keep images updated

Runtime Security:

• Drop capabilities
• Use read-only filesystem
• Set resource limits
• Enable security options
• Isolate networks
• Use secrets management

Compliance:

• Follow CIS Docker Benchmark
• Implement container scanning in CI/CD
• Use signed images (Docker Content Trust)
• Maintain audit logs
• Regular security reviews

Common Anti-Patterns to Avoid

❌ Don't:

• Run as root
• Use --privileged
• Mount Docker socket
• Use latest tag
• Hardcode secrets
• Skip health checks
• Ignore resource limits
• Use huge base images
• Skip vulnerability scanning
• Expose unnecessary ports
• Use inefficient layer caching
• Commit secrets to Git

✅ Do:

• Run as non-root
• Use minimal capabilities
• Isolate containers
• Tag with versions
• Use secrets management
• Implement health checks
• Set resource limits
• Use minimal images
• Scan regularly
• Apply least privilege
• Optimize build cache
• Use .env.example templates

Checklist for Production-Ready Images

• Based on official, versioned, minimal image
• Multi-stage build (if applicable)
• Runs as non-root user
• No secrets in layers
• .dockerignore configured
• Vulnerability scan passed
• Health check implemented
• Proper labeling (version, description, etc.)
• Efficient layer caching
• Resource limits defined
• Logging configured
• Signals handled correctly
• Security options set
• Documentation complete
• Tested on target platform(s)

This skill represents current Docker best practices. Always verify against official documentation for the latest recommendations, as Docker evolves continuously.