feat: Add container execution backend (Docker/Podman)

[germainh512]

Summary

Add support for running gvtest tests inside containers (Docker/Podman) while operating on the host filesystem via transparent bind mounts. This enables multi-project test suites where each project requires its own containerized environment, while keeping test artifacts directly accessible on the host.

Motivation

When integrating test suites from different projects, each may require its own set of toolchains, SDKs, and dependencies. Containers provide isolated environments, but traditionally require copying artifacts in and out. This feature lets gvtest orchestrate tests across multiple containers transparently — the container sees the same filesystem paths as the host, so build outputs, logs, and binaries are directly usable from either side.

Usage

Container config can be set at three levels (highest priority first):

1. In testset.cfg (programmatic)

def testset_build(testset):
    testset.set_container(
        image='ghcr.io/org/image:tag',
        setup='pip install -e .',
    )
    test = testset.new_test('build')
    test.add_command(Shell('build', 'make all'))

2. In gvtest.yaml (declarative, zero code changes)

container:
  image: ghcr.io/org/image:tag
  runtime: docker   # or podman
  setup: pip install -e .
  env:
    MY_VAR: value
  volumes:
    /extra/data: /extra/data
  options:
    - --gpus
    - all

3. Inherited from parent testsets

Child testsets inherit the container config from their parent unless they override it.

Key Design Decisions

• Transparent mounts: The test working directory is bind-mounted at the same absolute path inside the container (-v /path:/path), so all paths work identically on both sides
• Single docker run per test: All Shell commands within a test execute in one container invocation
• Works with pytest integration: Both test discovery (--collect-only) and batch execution run inside the container when configured; JUnit XML is written to the workdir (visible on both sides)
• Docker and Podman support: Configurable via runtime field
• Leaf-wins config precedence: In the gvtest.yaml hierarchy, the most specific (leaf-most) container config fully overrides parent configs (no merging)

Changes

File	Change
python/gvtest/container.py	New — ContainerConfig class with build_run_cmd(), from_dict(), validate()
python/gvtest/config.py	Parse and resolve container from gvtest.yaml hierarchy
python/gvtest/tests.py	Wrap Shell commands in docker run when container is configured
python/gvtest/pytest_integration.py	Container-aware discovery and batch execution
python/gvtest/testset_impl.py	set_container() and get_container() with parent inheritance
python/gvtest/testsuite.py	Abstract interface for container support
python/gvtest/runner.py	Auto-apply container config from gvtest.yaml on testset import
python/gvtest/__init__.py	Export ContainerConfig
tests/test_container.py	New — 20 unit tests

Testing

• All 156 tests pass (136 existing + 20 new)
• No regressions in existing functionality