Add Docker backend support for MicroPython execution without hardware

[Copilot]

This PR implements Docker backend support for micropython-magic, allowing users to run MicroPython code in Docker containers instead of requiring physical MCU hardware.

Key Changes

🐳 Docker Backend Implementation

• New DockerMicroPython class that interfaces with micropython/unix:latest Docker containers
• Modified MPRemote2 to support both serial and Docker backends via a backend parameter
• Enhanced magic commands with --backend docker and --docker-image options
• Container lifecycle management that automatically creates, starts, and reuses containers between magic calls

🎯 Usage Examples

Basic Docker execution:

%%micropython --backend docker
print("Hello from MicroPython in Docker!")
import sys
print(f"Running: {sys.version}")

Line magic with system info:

%micropython --backend docker --info

Custom Docker image:

%%micropython --backend docker --docker-image micropython/unix:latest
print("Using specific image version")

🔧 Backend Selection

• Seamless switching between --backend serial (default, existing behavior) and --backend docker
• Full backward compatibility - existing notebooks continue to work unchanged
• Graceful fallback when Docker is not available with clear error messages

📚 Documentation & Testing

• Comprehensive demo notebook (samples/docker_backend_demo.ipynb) showcasing all features
• Updated README with installation instructions and usage examples
• Complete test suite covering Docker backend functionality and integration
• Unit tests for both backend selection and container management

🎯 Benefits

• No hardware required - Perfect for learning, development, and CI/CD
• Consistent environment - Same MicroPython version across different machines
• Easy setup - Just requires Docker installation
• Safe experimentation - No risk of damaging physical hardware
• Educational friendly - Ideal for workshops and tutorials

🚧 Limitations

• No GPIO/hardware access (no machine module functionality)
• No hardware-specific features (WiFi, sensors, etc.)
• Variables don't persist between cells (each execution is independent)

The Docker backend uses the official micropython/unix:latest image and is ideal for algorithm development, code validation, and educational use cases before deploying to physical hardware.

Fixes #54.

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