fix: keep the harness admin password out of the docker exec argv

tests/integration/test_harness.py

@@ -157,3 +157,27 @@ def test_container_id_candidates_fall_back_to_proc(tmp_path: Path) -> None:
 
 def test_container_id_candidates_tolerate_missing_files(tmp_path: Path) -> None:
     assert harness.container_id_candidates({}, proc_root=tmp_path) == []
+
+
+def test_run_docker_passes_secrets_through_the_environment_not_argv(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    seen: dict[str, object] = {}
+
+    def fake_run(argv: list[str], **kwargs: object) -> object:
+        seen["argv"] = argv
+        seen["env"] = kwargs.get("env")
+
+        class Done:
+            returncode = 0
+            stdout = ""
+            stderr = ""
+
+        return Done()
+
+    monkeypatch.setattr(harness.subprocess, "run", fake_run)
+    harness.run_docker("exec", "-e", "SECRET_VAR", "box", env={"SECRET_VAR": "s3cret-value"})
+
+    assert "s3cret-value" not in " ".join(seen["argv"])  # type: ignore[call-overload]
+    assert seen["env"]["SECRET_VAR"] == "s3cret-value"  # type: ignore[index]
+    assert "PATH" in seen["env"]  # type: ignore[operator]