6.6Docker Troubleshooting for QA

Common Issues

Issue	Diagnosis	Fix
Container won't start	docker logs <container>	Check error messages in logs
Port already in use	lsof -i :<port>	Stop conflicting process or use different port
Container runs but app doesn't connect	docker network inspect	Ensure containers are on the same network
"No space left on device"	docker system df	Run docker system prune to clean up
Tests fail with connection refused	Check depends_on and healthchecks	Add healthcheck; wait for service readiness

Cleanup Commands

# Show disk usage by Docker
docker system df

# Remove all unused data (stopped containers, dangling images, unused networks)
docker system prune

# Remove all unused data including unused images (more aggressive)
docker system prune -a

# Remove specific types
docker container prune    # Stopped containers
docker image prune       # Dangling images
docker volume prune      # Unused volumes
docker network prune     # Unused networks