Docker Troubleshooting Skill

Overview

This skill helps diagnose and resolve common Docker issues:

• Container startup failures
• Networking problems
• Permission errors
• Port conflicts
• Data persistence issues
• Health check failures

Process

1. Consult Documentation

Read relevant documentation:

• 17-troubleshooting.md for common issues
• 15-port-conflicts.md for port problems
• 16-restart-strategies.md for restart issues

2. Diagnose Issue

Gather information:

# Check container status
docker compose ps -a

# View logs
docker compose logs servicename

# Check configuration
docker compose config

# Inspect container
docker inspect containername

3. Apply Solution

Common Issues and Solutions

Container Won't Start

Diagnosis:

docker compose logs servicename
docker inspect --format='{{.State.ExitCode}}' containername

Solutions:

• Check logs for error messages
• Verify configuration syntax
• Check dependencies are running
• Verify image exists

Port Already in Use

Diagnosis:

lsof -i :3000
# or
netstat -tulpn | grep 3000

Solutions:

# Kill process
kill $(lsof -t -i:3000)

# Or change port in compose
ports:
  - "3001:3000"

Permission Denied

Diagnosis:

docker compose exec app ls -la /app/data

Solutions:

# Fix in compose
services:
  app:
    user: "1000:1000"

# Or fix in container
docker compose exec -u root app chown -R appuser:appgroup /app/data

Data Disappearing

Diagnosis:

docker volume ls
docker compose config | grep -A5 "volumes:"

Solutions:

# Use named volumes instead of anonymous
volumes:
  - postgres_data:/var/lib/postgresql/data  # Named (persists)
  # NOT: - /var/lib/postgresql/data  # Anonymous (deleted)

Container Can't Reach Other Container

Diagnosis:

docker network inspect networkname
docker compose exec app ping db
docker compose exec app nslookup db

Solutions:

# Ensure same network
services:
  app:
    networks:
      - backend
  db:
    networks:
      - backend

networks:
  backend:

Health Check Failing

Diagnosis:

docker inspect --format='{{json .State.Health}}' containername | jq

Solutions:

healthcheck:
  test: ["CMD", "curl", "-f", "http://localhost:3000/health"]
  interval: 30s
  timeout: 10s
  retries: 3
  start_period: 40s  # Give time to start

Out of Disk Space

Diagnosis:

docker system df
docker system df -v

Solutions:

# Clean unused resources
docker system prune -a --volumes

Build Cache Issues

Solutions:

docker compose build --no-cache
docker builder prune -a

Debugging Commands

# Shell into running container
docker compose exec app sh

# Shell into failed container
docker compose run --entrypoint sh app

# View resource usage
docker stats

# View container processes
docker compose top

# View real-time events
docker events

# Copy files from container
docker compose cp app:/app/logs ./logs

Quick Diagnostic Checklist

1. Check if container is running: docker compose ps
2. Check logs: docker compose logs servicename
3. Check network: docker network ls
4. Check volumes: docker volume ls
5. Check resources: docker stats
6. Validate config: docker compose config
7. Check disk space: docker system df

Error Messages Reference

Error	Cause	Solution
"port is already allocated"	Port in use	Kill process or change port
"network not found"	Missing network	Create network or check name
"volume not found"	Missing volume	Create volume or check name
"no such service"	Service name typo	Check compose.yaml
"unauthorized"	Auth issue	docker login
"image not found"	Missing image	docker compose pull
"permission denied"	File permissions	Fix ownership/permissions
"out of memory"	Memory limit	Increase limit or optimize app