This commit adds Docker backend support to both the TypeScript and Python SDKs:

TypeScript SDK:
- Add BackendType type for backend selection (wasmer, firecracker, docker, auto)
- Add DockerOptions interface for Docker-specific configuration
- Add backend option to BashletOptions and ExecOptions
- Update client to pass --backend flag to CLI
- Export new types from index
- Add tests for Docker backend functionality

Python SDK:
- Add BackendType enum for backend selection
- Add DockerOptions dataclass for Docker-specific configuration
- Add backend option to BashletOptions and ExecOptions
- Update both sync and async clients to support backend parameter
- Support both string and enum values for backend
- Export new types from __init__.py
- Add tests for Docker backend functionality

Users can now specify the Docker backend when executing commands:

TypeScript:
  const bashlet = new Bashlet({ backend: 'docker' });
  await bashlet.exec('ls', { backend: 'docker' });

Python:
  bashlet = Bashlet(backend='docker')
  bashlet.exec('ls', backend=BackendType.DOCKER)