[env] feat: add local Docker sandbox provider

[yyDing1]

Summary

Add a Docker-backed sandbox provider for running agents in isolated local containers.

Changes

• Add DockerSandbox with container lifecycle management.
• Execute commands through docker exec.
• Transfer files through docker cp.
• Support working directories and environment variables.
• Support always, missing, and never image pull policies.
• Register the docker sandbox provider.
• Add Docker configuration and quickstart documentation.
• Add lifecycle, configuration, execution, and file-transfer tests.

Example

sandbox:
  provider: docker
  image: python:3.12
  sandbox_kwargs:
    pull_policy: missing

[gemini-code-assist]

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