docker

name	docker
description	Container-based development for isolated, reproducible environments. Use when running npm commands, installing packages, executing code, or managing project dependencies. Trigger phrases include "npm install", "run the build", "start the server", "install package", or any code execution request.

Docker Development Skill

Execute all package installations and code execution inside Docker containers. This keeps the host machine clean and ensures consistent environments across projects.

Core Principle

NEVER run npm, node, npx, or project scripts directly on the host machine.

Instead, use docker exec or ensure the container is running the dev server.

Pre-Flight Check (MANDATORY)

Before running ANY npm/node command, Claude Code MUST verify the container is running.

Run this check first:

docker ps --filter "name=<project-name>" --format "table {{.Names}}\t{{.Status}}\t{{.Ports}}"

Expected output:

NAMES                  STATUS          PORTS
<project-name>-dev-1   Up X minutes    0.0.0.0:3000->3000/tcp

If container is NOT running:

# Navigate to project root first
cd /path/to/your/project

# Start container
docker-compose --profile dev up dev -d

# Verify it started
docker ps --filter "name=<project-name>"

If container shows "Exited":

# Check why it exited
docker logs <project-name>-dev-1 --tail 20

# Remove and restart
docker-compose --profile dev down
docker-compose --profile dev up dev -d

Quick Reference

Check Container Status

# List running containers for current project
docker ps --filter "name=<project-name>"

# Check container logs
docker logs <project-name>-dev-1 --tail 50

# Check if dev server is responding
curl -s http://localhost:3000 > /dev/null && echo "Server running" || echo "Server not running"

Start/Stop Containers

# Start development container (from project root)
docker-compose --profile dev up dev -d

# Stop container
docker-compose --profile dev down

# Restart container
docker-compose --profile dev restart dev

# Rebuild after Dockerfile changes
docker-compose --profile dev up dev -d --build

Execute Commands Inside Container

# Install a package
docker exec -it <project-name>-dev-1 npm install <package-name>

# Install dev dependency
docker exec -it <project-name>-dev-1 npm install -D <package-name>

# Run tests
docker exec -it <project-name>-dev-1 npm test

# Run type checking
docker exec -it <project-name>-dev-1 npm run typecheck

# Run linting
docker exec -it <project-name>-dev-1 npm run lint

# Run build
docker exec -it <project-name>-dev-1 npm run build

# Open shell inside container
docker exec -it <project-name>-dev-1 /bin/sh

# Run any arbitrary command
docker exec -it <project-name>-dev-1 <command>

When to Use Docker exec

Operation	Use docker exec?	Reason
npm install	✅ Yes	Packages install in container
npm run dev	❌ No	Already running via docker-compose
npm test	✅ Yes	Tests run in container environment
npm run build	✅ Yes	Build happens in container
git commands	❌ No	Git runs on host (manages files)
File editing	❌ No	Volume mount syncs automatically
Database migrations	✅ Yes	Uses container's Node environment

Container Architecture

┌─────────────────────────────────────────────────────────────┐
│  HOST (macOS/Linux/Windows)                                 │
│                                                             │
│  ┌─────────────────────────────────────────────────────┐   │
│  │  Docker Container (<project-name>-dev-1)            │   │
│  │                                                     │   │
│  │  Node 20 Alpine                                     │   │
│  │  └── node_modules/ (container-only)                 │   │
│  │  └── Dev server (port 3000)                         │   │
│  │                                                     │   │
│  │  Volume Mounts:                                     │   │
│  │  └── .:/app (source code sync)                      │   │
│  │  └── node_modules:/app/node_modules (persist deps)  │   │
│  └─────────────────────────────────────────────────────┘   │
│                         │                                   │
│                         ▼                                   │
│                   Port 3000 mapped                          │
│                         │                                   │
│                         ▼                                   │
│              http://localhost:3000                          │
└─────────────────────────────────────────────────────────────┘

Volume Mount Behavior

The docker-compose.yml mounts the project directory into the container:

volumes:
  - .:/app                           # Source code (synced)
  - /app/node_modules                # Dependencies (container-only)

What this means:

• Source code changes on host are immediately visible in container
• node_modules/ in container is separate from any on host
• Hot reload works automatically with most frameworks

Troubleshooting

Container Not Running

# Check if container exists
docker ps -a --filter "name=<project-name>"

# If exited, check why
docker logs <project-name>-dev-1

# Restart
docker-compose --profile dev up dev -d

Port Already in Use

# Find what's using the port
lsof -i :3000

# Kill the process or change port in docker-compose.yml

Module Not Found Errors

# Rebuild container with fresh dependencies
docker-compose --profile dev down
docker-compose --profile dev build --no-cache dev
docker-compose --profile dev up dev -d

File Changes Not Reflecting

# Check volume mounts
docker inspect <project-name>-dev-1 | grep -A 10 "Mounts"

# Restart container
docker-compose --profile dev restart dev

Project Configuration

After installing this skill, update the placeholders for your project:

Setting	Example Value
Container name	my-app-dev-1
Port	3000 (or your app's port)
Node version	20 (Alpine)
Dev command	npm run dev -- --host 0.0.0.0

Environment Variables

Required env vars are loaded from .env file via docker-compose.

If a command needs a specific env var:

docker exec -it -e MY_VAR=value <project-name>-dev-1 <command>

Best Practices

1. Always check container status before running commands
2. Use docker exec for all npm/node operations
3. Let volume mounts handle file syncing (no manual copying)
4. Rebuild image after changing package.json or Dockerfile
5. Check logs if something isn't working

Integration with Claude Code

When Claude Code needs to:

Task	Action
Install dependency	docker exec -it <project-name>-dev-1 npm install <pkg>
Run tests	docker exec -it <project-name>-dev-1 npm test
Check types	docker exec -it <project-name>-dev-1 npm run typecheck
Build project	docker exec -it <project-name>-dev-1 npm run build
Start dev server	Container already runs it via docker-compose
Edit files	Edit directly (volume mount syncs)
Git operations	Run on host (not in container)

Sample docker-compose.yml

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
    env_file:
      - .env
    restart: unless-stopped

  dev:
    build:
      context: .
      dockerfile: Dockerfile.dev
    ports:
      - "3000:3000"
    volumes:
      - .:/app
      - /app/node_modules
    environment:
      - NODE_ENV=development
    env_file:
      - .env
    profiles:
      - dev

Sample Dockerfile.dev

FROM node:20-alpine

WORKDIR /app

# Install dependencies for native modules
RUN apk add --no-cache python3 make g++

# Copy package files and any scripts needed for postinstall
COPY package*.json ./
COPY scripts/ ./scripts/

# Install all dependencies
RUN npm install

# Copy source code
COPY . .

# Expose port
EXPOSE 3000

# Set environment variables
ENV HOST=0.0.0.0
ENV PORT=3000
ENV NODE_ENV=development

# Start development server
CMD ["npm", "run", "dev", "--", "--host", "0.0.0.0"]