Add Memory & CPU Quota Limits to DockerSandbox

[Kaap10]

Description

When evaluating coding agents with DockerSandbox, an agent's code could consume excessive memory or CPU (e.g., through infinite loops, recursion, or memory leaks). DockerSandbox should enforce configurable resource quotas.

Proposed Solution

1. Add DOCKER_MEMORY_LIMIT (default "2g") and DOCKER_CPU_QUOTA (default 1.0) settings to backend/app/config/settings.py.
2. Update backend/app/sandbox/docker_sandbox.py to pass mem_limit and nano_cpus parameters when spawning Docker containers via docker.containers.run().
3. Add a unit test verifying container creation arguments.

Relevant Files

• backend/app/sandbox/docker_sandbox.py
• backend/app/config/settings.py

Acceptance Criteria

• Containers spawned by DockerSandbox enforce configured memory and CPU limits.
• Configurable via .env environment variables.
• Unit tests pass with python -m pytest.