test(docker): add conftest fixtures for docker harness

Task 0.1 of the s6-overlay supervision plan. Establishes the test
infrastructure for tests/docker/: skip-on-missing-Docker collection
hook, session-scoped image-build fixture (overridable via the
HERMES_TEST_IMAGE env var for faster local iteration), and a
container_name fixture that ensures cleanup on test exit.

Refs: docs/plans/2026-05-07-s6-overlay-dynamic-subagent-gateways.md

tests/docker/conftest.py

@@ -0,0 +1,79 @@
+"""Shared fixtures for docker-image integration tests.
+
+Tests in this directory build the image with the current ``Dockerfile``
+and exercise it via ``docker run``. They skip when Docker is unavailable
+(e.g. on developer laptops without a daemon).
+
+Override the image with ``HERMES_TEST_IMAGE`` env var to point at a pre-built
+image (faster local iteration); otherwise the ``built_image`` fixture builds
+the repo's Dockerfile once per session.
+"""
+from __future__ import annotations
+
+import os
+import shutil
+import subprocess
+from collections.abc import Iterator
+
+import pytest
+
+IMAGE_TAG = os.environ.get("HERMES_TEST_IMAGE", "hermes-agent-harness:latest")
+
+
+def _docker_available() -> bool:
+    """Return True iff a docker CLI is on PATH and the daemon answers."""
+    if shutil.which("docker") is None:
+        return False
+    try:
+        r = subprocess.run(
+            ["docker", "info"], capture_output=True, timeout=5,
+        )
+        return r.returncode == 0
+    except (subprocess.TimeoutExpired, OSError):
+        return False
+
+
+def pytest_collection_modifyitems(config, items):  # noqa: D401 - pytest hook
+    """Skip every test under tests/docker/ when docker is unavailable."""
+    if _docker_available():
+        return
+    skip_docker = pytest.mark.skip(
+        reason="Docker not available or daemon not running",
+    )
+    for item in items:
+        if "tests/docker/" in str(item.fspath).replace(os.sep, "/"):
+            item.add_marker(skip_docker)
+
+
+@pytest.fixture(scope="session")
+def built_image() -> str:
+    """Build the image once per test session.
+
+    Override with ``HERMES_TEST_IMAGE`` env var to point at a pre-built
+    image (faster local iteration).
+    """
+    if os.environ.get("HERMES_TEST_IMAGE"):
+        return IMAGE_TAG
+    repo_root = os.path.abspath(
+        os.path.join(os.path.dirname(__file__), "..", ".."),
+    )
+    result = subprocess.run(
+        ["docker", "build", "-t", IMAGE_TAG, repo_root],
+        capture_output=True, text=True, timeout=1200,
+    )
+    assert result.returncode == 0, (
+        f"docker build failed:\n{result.stderr[-2000:]}"
+    )
+    return IMAGE_TAG
+
+
+@pytest.fixture
+def container_name(request) -> Iterator[str]:
+    """Generate a unique container name and ensure cleanup on test exit."""
+    safe = request.node.name.replace("[", "_").replace("]", "_")
+    name = f"hermes-test-{safe}"
+    yield name
+    subprocess.run(
+        ["docker", "rm", "-f", name],
+        capture_output=True, timeout=10,
+    )