# Docker Troubleshooting Reference

## Container Won't Start

### Check logs
```bash
docker logs <container_name>
docker logs --tail 50 -f <container_name>
```

### Common issues
- **Port already in use**: `docker ps` to find conflicting containers
- **Permission errors**: Check file ownership and container user
- **Missing environment variables**: Verify with `docker exec -it <container> env`
- **Resource limits**: Check memory/CPU constraints

### Debug running container
```bash
docker exec -it <container> /bin/bash
docker exec -it <container> /bin/sh  # if bash not available
```

## Build Issues

### Clear build cache
```bash
docker system prune -a
docker builder prune
```

### Build with verbose output
```bash
docker build --progress=plain --no-cache .
```

### Common build failures
- **COPY/ADD errors**: Check file paths and .dockerignore
- **Package installation**: Update package lists first
- **Network issues**: Check DNS and proxy settings

## Performance Issues

### Container resource usage
```bash
docker stats
docker exec <container> top
docker exec <container> free -h
```

### Image size optimization
```bash
docker images --format "table {{.Repository}}\t{{.Tag}}\t{{.Size}}"
docker history <image>
```

## Network Debugging

### List networks and connections
```bash
docker network ls
docker network inspect <network_name>
docker port <container>
```

### Test connectivity
```bash
docker exec <container> ping <target>
docker exec <container> nslookup <hostname>
docker exec <container> netstat -tulpn
```

## Quick Fixes

### Remove all stopped containers
```bash
docker container prune
```

### Remove unused images
```bash
docker image prune -a
```

### Reset Docker completely
```bash
docker system prune -a --volumes
```