Docker Skill

This skill provides comprehensive guidance for working with Docker, covering containerization concepts, practical workflows, and best practices across all major technology stacks.

When to Use This Skill

Use this skill when:

Containerizing applications for any language or framework
Creating or optimizing Dockerfiles and Docker Compose configurations
Setting up development environments with Docker
Deploying containerized applications to production
Implementing CI/CD pipelines with Docker
Managing container networking, storage, and security
Troubleshooting Docker-related issues
Building multi-platform images
Implementing microservices architectures

Core Docker Concepts

Containers

Lightweight, isolated processes that bundle applications with all dependencies
Provide filesystem isolation via union filesystems and namespace technology
Ephemeral by default - changes are lost when container stops (unless persisted to volumes)
Single responsibility principle: each container should do one thing well
Multiple identical containers can run from same immutable image without conflicts

Images

Blueprint/template for containers - read-only filesystems + configuration
Composed of layered filesystem (immutable, reusable layers)
Built from Dockerfile instructions or committed from running containers
Stored in registries (Docker Hub, ECR, ACR, GCR, private registries)

Image naming:

REGISTRY/NAMESPACE/REPOSITORY:TAG

(e.g.,

docker.io/library/nginx:latest

)

Volumes & Storage

Volumes: Docker-managed persistent storage that survives container deletion
Bind mounts: Direct mapping of host filesystem paths into containers
tmpfs mounts: In-memory storage for temporary data
Enable data sharing between containers and persist beyond container lifecycle

Networks

Default bridge network connects containers on same host
Custom networks allow explicit container communication with DNS resolution
Host network removes network isolation for performance
Overlay networks enable multi-host container communication (Swarm)
MACVLAN/IPvlan for containers needing direct L2/L3 network access

Dockerfile Best Practices

Essential Instructions

dockerfile

FROM <image>:<tag>                        # Base image (use specific versions, not 'latest')
WORKDIR /app                              # Working directory for subsequent commands
COPY package*.json ./                     # Copy dependency files first (for caching)
RUN npm install --production              # Execute build commands
COPY . .                                  # Copy application code
ENV NODE_ENV=production                   # Environment variables
EXPOSE 3000                               # Document exposed ports
USER node                                 # Run as non-root user (security)
CMD ["node", "server.js"]                 # Default command when container starts

Multi-Stage Builds (Critical for Production)

Separate build environment from runtime environment to reduce image size and improve security:

dockerfile

# Stage 1: Build
FROM node:20-alpine AS build
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
RUN npm run build

# Stage 2: Production
FROM node:20-alpine AS production
WORKDIR /app
COPY --from=build /app/dist ./dist
COPY --from=build /app/node_modules ./node_modules
USER node
EXPOSE 3000
CMD ["node", "dist/server.js"]

Benefits: Compiled assets without build tools in final image, smaller size, improved security

Layer Caching Optimization

Order matters! Docker reuses layers if instruction unchanged:

Dependencies first (COPY package.json, RUN npm install)
Application code last (COPY . .)
This way, code changes don't invalidate dependency layers

Security Hardening

dockerfile

# Use specific versions
FROM node:20.11.0-alpine3.19

# Create non-root user
RUN addgroup -g 1001 -S nodejs && \
    adduser -S nodejs -u 1001

# Set ownership
COPY --chown=nodejs:nodejs . .

# Switch to non-root
USER nodejs

# Read-only root filesystem (when possible)
# Add --read-only flag when running container

.dockerignore File

Exclude unnecessary files from build context:

node_modules
.git
.env
.env.local
*.log
.DS_Store
README.md
docker-compose.yml
.dockerignore
Dockerfile
dist
coverage
.vscode

Common Workflows

Building Images

bash

# Build with tag
docker build -t myapp:1.0 .

# Build targeting specific stage
docker build -t myapp:dev --target build .

# Build with build arguments
docker build --build-arg NODE_ENV=production -t myapp:1.0 .

# Build for multiple platforms
docker buildx build --platform linux/amd64,linux/arm64 -t myapp:1.0 .

# View image layers and size
docker image history myapp:1.0

# List all images
docker image ls

Running Containers

bash

# Basic run
docker run myapp:1.0

# Run in background (detached)
docker run -d --name myapp myapp:1.0

# Port mapping (host:container)
docker run -p 8080:3000 myapp:1.0

# Environment variables
docker run -e NODE_ENV=production -e API_KEY=secret myapp:1.0

# Volume mount (named volume)
docker run -v mydata:/app/data myapp:1.0

# Bind mount (development)
docker run -v $(pwd)/src:/app/src myapp:1.0

# Custom network
docker run --network my-network myapp:1.0

# Resource limits
docker run --memory 512m --cpus 0.5 myapp:1.0

# Interactive terminal
docker run -it myapp:1.0 /bin/sh

# Override entrypoint/command
docker run --entrypoint /bin/sh myapp:1.0
docker run myapp:1.0 custom-command --arg

Container Management

bash

# List running containers
docker ps

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

# View logs
docker logs myapp
docker logs -f myapp              # Follow logs
docker logs --tail 100 myapp      # Last 100 lines

# Execute command in running container
docker exec myapp ls /app
docker exec -it myapp /bin/sh     # Interactive shell

# Stop container (graceful)
docker stop myapp

# Kill container (immediate)
docker kill myapp

# Remove container
docker rm myapp
docker rm -f myapp                # Force remove running container

# View container details
docker inspect myapp

# Monitor resource usage
docker stats myapp

# View container processes
docker top myapp

# Copy files to/from container
docker cp myapp:/app/logs ./logs
docker cp ./config.json myapp:/app/config.json

Image Management

bash

# Tag image
docker tag myapp:1.0 registry.example.com/myapp:1.0

# Push to registry
docker login registry.example.com
docker push registry.example.com/myapp:1.0

# Pull from registry
docker pull nginx:alpine

# Remove image
docker image rm myapp:1.0

# Remove unused images
docker image prune

# Remove all unused resources (images, containers, volumes, networks)
docker system prune -a

# View disk usage
docker system df

Volume Management

bash

# Create named volume
docker volume create mydata

# List volumes
docker volume ls

# Inspect volume
docker volume inspect mydata

# Remove volume
docker volume rm mydata

# Remove unused volumes
docker volume prune

Network Management

bash

# Create network
docker network create my-network
docker network create --driver bridge my-bridge

# List networks
docker network ls

# Inspect network
docker network inspect my-network

# Connect container to network
docker network connect my-network myapp

# Disconnect container from network
docker network disconnect my-network myapp

# Remove network
docker network rm my-network

Docker Compose

When to Use Compose

Multi-container applications (web + database + cache)
Consistent development environments across team
Simplifying complex docker run commands
Managing application dependencies and startup order

Basic Compose File Structure

yaml

version: '3.8'

services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
      - DATABASE_URL=postgresql://user:pass@db:5432/app
    depends_on:
      - db
      - redis
    volumes:
      - ./src:/app/src      # Development: live code reload
    networks:
      - app-network
    restart: unless-stopped

  db:
    image: postgres:15-alpine
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: app
    volumes:
      - postgres_data:/var/lib/postgresql/data
    networks:
      - app-network
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user"]
      interval: 10s
      timeout: 5s
      retries: 5

  redis:
    image: redis:7-alpine
    networks:
      - app-network
    volumes:
      - redis_data:/data

volumes:
  postgres_data:
  redis_data:

networks:
  app-network:
    driver: bridge

Compose Commands

bash

# Start all services
docker compose up

# Start in background
docker compose up -d

# Build images before starting
docker compose up --build

# Scale specific service
docker compose up -d --scale web=3

# Stop all services
docker compose down

# Stop and remove volumes
docker compose down --volumes

# View logs
docker compose logs
docker compose logs -f web        # Follow specific service

# Execute command in service
docker compose exec web sh
docker compose exec db psql -U user -d app

# List running services
docker compose ps

# Restart service
docker compose restart web

# Pull latest images
docker compose pull

# Validate compose file
docker compose config

Development vs Production Compose

compose.yml (base configuration):

yaml

services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - DATABASE_URL=postgresql://user:pass@db:5432/app

compose.override.yml (development overrides, loaded automatically):

yaml

services:
  web:
    volumes:
      - ./src:/app/src      # Live code reload
    environment:
      - NODE_ENV=development
      - DEBUG=true
    command: npm run dev

compose.prod.yml (production overrides):

yaml

services:
  web:
    image: registry.example.com/myapp:1.0
    restart: always
    environment:
      - NODE_ENV=production
    deploy:
      replicas: 3
      resources:
        limits:
          cpus: '0.5'
          memory: 512M

Usage:

bash

# Development (uses compose.yml + compose.override.yml automatically)
docker compose up

# Production (explicit override)
docker compose -f compose.yml -f compose.prod.yml up -d

Language-Specific Dockerfiles

Node.js

dockerfile

FROM node:20-alpine AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
RUN npm run build

FROM node:20-alpine AS production
WORKDIR /app
COPY --from=build /app/dist ./dist
COPY --from=build /app/node_modules ./node_modules
COPY package*.json ./
USER node
EXPOSE 3000
CMD ["node", "dist/server.js"]

Python

dockerfile

FROM python:3.11-slim AS build
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

FROM python:3.11-slim AS production
WORKDIR /app
COPY --from=build /usr/local/lib/python3.11/site-packages /usr/local/lib/python3.11/site-packages
COPY . .
RUN adduser --disabled-password --gecos '' appuser && \
    chown -R appuser:appuser /app
USER appuser
EXPOSE 8000
CMD ["python", "app.py"]

Go

dockerfile

FROM golang:1.21-alpine AS build
WORKDIR /app
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 GOOS=linux go build -o main .

FROM scratch
COPY --from=build /app/main /main
EXPOSE 8080
CMD ["/main"]

Java (Spring Boot)

dockerfile

FROM eclipse-temurin:21-jdk-alpine AS build
WORKDIR /app
COPY pom.xml .
COPY src ./src
RUN ./mvnw clean package -DskipTests

FROM eclipse-temurin:21-jre-alpine AS production
WORKDIR /app
COPY --from=build /app/target/*.jar app.jar
RUN addgroup -g 1001 -S spring && \
    adduser -S spring -u 1001
USER spring
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "app.jar"]

React/Vue/Angular (Static SPA)

dockerfile

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

FROM nginx:alpine AS production
COPY --from=build /app/dist /usr/share/nginx/html
COPY nginx.conf /etc/nginx/nginx.conf
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

Production Deployment

Health Checks

In Dockerfile:

dockerfile

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

In Compose:

yaml

services:
  web:
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:3000/health"]
      interval: 30s
      timeout: 3s
      start-period: 40s
      retries: 3

Resource Limits

yaml

services:
  web:
    deploy:
      resources:
        limits:
          cpus: '0.5'
          memory: 512M
        reservations:
          cpus: '0.25'
          memory: 256M

Restart Policies

yaml

services:
  web:
    restart: unless-stopped    # Restart unless manually stopped
    # Other options: "no", "always", "on-failure"

Logging Configuration

yaml

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

Environment Variables & Secrets

Using .env file:

bash

# .env
DATABASE_URL=postgresql://user:pass@db:5432/app
API_KEY=secret

yaml

services:
  web:
    env_file:
      - .env

Using Docker secrets (Swarm):

yaml

services:
  web:
    secrets:
      - db_password

secrets:
  db_password:
    external: true

Production Checklist

✅ Use specific image versions (not
```
latest
```
)
✅ Run as non-root user
✅ Multi-stage builds to minimize image size
✅ Health checks implemented
✅ Resource limits configured
✅ Restart policy set
✅ Logging configured
✅ Secrets managed securely (not in environment variables)
✅ Vulnerability scanning (Docker Scout)
✅ Read-only root filesystem when possible
✅ Network segmentation
✅ Regular image updates

CI/CD Integration

GitHub Actions Example

yaml

name: Docker Build and Push

on:
  push:
    branches: [ main ]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3

      - name: Login to Docker Hub
        uses: docker/login-action@v3
        with:
          username: ${{ secrets.DOCKER_USERNAME }}
          password: ${{ secrets.DOCKER_PASSWORD }}

      - name: Build and push
        uses: docker/build-push-action@v5
        with:
          context: .
          push: true
          tags: user/app:latest,user/app:${{ github.sha }}
          cache-from: type=registry,ref=user/app:buildcache
          cache-to: type=registry,ref=user/app:buildcache,mode=max

      - name: Run vulnerability scan
        uses: docker/scout-action@v1
        with:
          command: cves
          image: user/app:${{ github.sha }}

Security Best Practices

Scan for Vulnerabilities

bash

# Using Docker Scout
docker scout cves myapp:1.0
docker scout recommendations myapp:1.0

# Quick view
docker scout quickview myapp:1.0

Run Containers Securely

bash

# Read-only root filesystem
docker run --read-only -v /tmp --tmpfs /run myapp:1.0

# Drop all capabilities, add only needed ones
docker run --cap-drop=ALL --cap-add=NET_BIND_SERVICE myapp:1.0

# No new privileges
docker run --security-opt=no-new-privileges myapp:1.0

# Use security profiles
docker run --security-opt apparmor=docker-default myapp:1.0

# Limit resources
docker run --memory=512m --cpus=0.5 --pids-limit=100 myapp:1.0

Image Security Checklist

✅ Start with minimal base images (Alpine, Distroless)
✅ Use specific versions, not
```
latest
```
✅ Scan for vulnerabilities regularly
✅ Run as non-root user
✅ Don't include secrets in images (use runtime secrets)
✅ Minimize attack surface (only install needed packages)
✅ Use multi-stage builds (no build tools in final image)
✅ Sign and verify images
✅ Keep images updated

Networking Patterns

Bridge Network (Default)

bash

# Create custom bridge network
docker network create my-bridge

# Run containers on custom bridge
docker run -d --name web --network my-bridge nginx
docker run -d --name db --network my-bridge postgres

# Containers can communicate via container name
# web can connect to: http://db:5432

Container Communication

yaml

services:
  web:
    depends_on:
      - db
    environment:
      # Use service name as hostname
      - DATABASE_URL=postgresql://user:pass@db:5432/app

  db:
    image: postgres:15-alpine

Port Publishing

bash

# Publish single port
docker run -p 8080:80 nginx

# Publish range of ports
docker run -p 8080-8090:8080-8090 myapp

# Publish to specific interface
docker run -p 127.0.0.1:8080:80 nginx

# Publish all exposed ports to random ports
docker run -P nginx

Storage Patterns

Named Volumes (Recommended for Data)

bash

# Create and use named volume
docker volume create app-data
docker run -v app-data:/app/data myapp

# Automatic creation
docker run -v app-data:/app/data myapp  # Creates if doesn't exist

Bind Mounts (Development)

bash

# Live code reload during development
docker run -v $(pwd)/src:/app/src myapp

# Read-only bind mount
docker run -v $(pwd)/config:/app/config:ro myapp

tmpfs Mounts (Temporary In-Memory)

bash

# Store temporary data in memory
docker run --tmpfs /tmp myapp

Volume Backup & Restore

bash

# Backup volume
docker run --rm -v app-data:/data -v $(pwd):/backup alpine \
  tar czf /backup/backup.tar.gz /data

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

Troubleshooting

Debug Running Container

bash

# View logs
docker logs -f myapp
docker logs --tail 100 myapp

# Interactive shell
docker exec -it myapp /bin/sh

# Inspect container
docker inspect myapp

# View processes
docker top myapp

# Monitor resource usage
docker stats myapp

# View changes to filesystem
docker diff myapp

Debug Build Issues

bash

# Build with verbose output
docker build --progress=plain -t myapp .

# Build specific stage for testing
docker build --target build -t myapp:build .

# Run failed build stage
docker run -it myapp:build /bin/sh

# Check build context
docker build --no-cache -t myapp .

Common Issues

Container exits immediately:

bash

# Check logs
docker logs myapp

# Run with interactive shell
docker run -it myapp /bin/sh

# Override entrypoint
docker run -it --entrypoint /bin/sh myapp

Cannot connect to container:

bash

# Check port mapping
docker ps
docker port myapp

# Check network
docker network inspect bridge
docker inspect myapp | grep IPAddress

# Check if service is listening
docker exec myapp netstat -tulpn

Out of disk space:

bash

# Check disk usage
docker system df

# Clean up
docker system prune -a
docker volume prune
docker image prune -a

Build cache issues:

bash

# Force rebuild without cache
docker build --no-cache -t myapp .

# Clear build cache
docker builder prune

Advanced Topics

Multi-Platform Builds

bash

# Setup buildx
docker buildx create --use

# Build for multiple platforms
docker buildx build --platform linux/amd64,linux/arm64 \
  -t myapp:1.0 --push .

Build Optimization

bash

# Use BuildKit (enabled by default in recent versions)
DOCKER_BUILDKIT=1 docker build -t myapp .

# Use build cache from registry
docker build --cache-from myapp:latest -t myapp:1.0 .

# Export cache to registry
docker build --cache-to type=registry,ref=myapp:buildcache \
  --cache-from type=registry,ref=myapp:buildcache \
  -t myapp:1.0 .

Docker Contexts

bash

# List contexts
docker context ls

# Create remote context
docker context create remote --docker "host=ssh://user@remote"

# Use context
docker context use remote
docker ps  # Now runs on remote host

# Switch back to default
docker context use default

Quick Reference

Most Common Commands

Task	Command
Build image	`docker build -t myapp:1.0 .`
Run container	`docker run -d -p 8080:3000 myapp:1.0`
View logs	`docker logs -f myapp`
Shell into container	`docker exec -it myapp /bin/sh`
Stop container	`docker stop myapp`
Remove container	`docker rm myapp`
Start Compose	`docker compose up -d`
Stop Compose	`docker compose down`
View Compose logs	`docker compose logs -f`
Clean up all	`docker system prune -a`

Recommended Base Images

Language/Framework	Recommended Base
Node.js	`node:20-alpine`
Python	`python:3.11-slim`
Java	`eclipse-temurin:21-jre-alpine`
Go	`scratch` (for compiled binary)
.NET	`mcr.microsoft.com/dotnet/aspnet:8.0-alpine`
PHP	`php:8.2-fpm-alpine`
Ruby	`ruby:3.2-alpine`
Static sites	`nginx:alpine`

Additional Resources

Official Documentation: https://docs.docker.com
Docker Hub: https://hub.docker.com (public image registry)
Best Practices: https://docs.docker.com/develop/dev-best-practices/
Security: https://docs.docker.com/engine/security/
Dockerfile Reference: https://docs.docker.com/engine/reference/builder/
Compose Specification: https://docs.docker.com/compose/compose-file/

Summary

Docker containerization provides:

Consistency across development, testing, and production
Isolation for applications and dependencies
Portability across different environments
Efficiency through layered architecture and caching
Scalability for microservices and distributed systems

Follow multi-stage builds, run as non-root, use specific versions, implement health checks, scan for vulnerabilities, and configure resource limits for production-ready containerized applications.

docker

NPX Install

Tags

SKILL.md Content

Docker Skill

When to Use This Skill

Core Docker Concepts

Containers

Images

Volumes & Storage

Networks

Dockerfile Best Practices

Essential Instructions

Multi-Stage Builds (Critical for Production)

Layer Caching Optimization

Security Hardening

.dockerignore File

Common Workflows

Building Images

Running Containers

Container Management

Image Management

Volume Management

Network Management

Docker Compose

When to Use Compose

Basic Compose File Structure

Compose Commands

Development vs Production Compose

Language-Specific Dockerfiles

Node.js

Python

Go

Java (Spring Boot)

React/Vue/Angular (Static SPA)

Production Deployment

Health Checks

Resource Limits

Restart Policies

Logging Configuration

Environment Variables & Secrets

Production Checklist

CI/CD Integration

GitHub Actions Example

Security Best Practices

Scan for Vulnerabilities

Run Containers Securely

Image Security Checklist

Networking Patterns

Bridge Network (Default)

Container Communication

Port Publishing

Storage Patterns

Named Volumes (Recommended for Data)

Bind Mounts (Development)

tmpfs Mounts (Temporary In-Memory)

Volume Backup & Restore

Troubleshooting

Debug Running Container

Debug Build Issues

Common Issues

Advanced Topics

Multi-Platform Builds

Build Optimization

Docker Contexts

Quick Reference

Most Common Commands

Recommended Base Images

Additional Resources

Summary