DEVX-155: fix: trust /var/run/docker.sock with free=0 when no inner dockerd exists

The host's rootless Docker socket is mounted as /var/run/docker.sock
inside CI containers. Its data root is on the host filesystem (not
accessible from inside the container), so _get_docker_free_bytes
returns 0. Previously the code didn't trust this and started a local
dockerd on /dev/shm (too small). Now checks pgrep for dockerd processes
— if none found inside the container, the socket is the host's Docker
and should be trusted.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_start_docker.py

@@ -205,8 +205,10 @@ class TestStartDockerDaemon:
         mock_ntf.return_value = MagicMock(name="/tmp/dockerd.log")
         # is_docker_ready: first check (inner dockerd) True, then local daemon checks
         mock_ready.side_effect = [True, False, False, False, False, True]
-        # subprocess.run calls: pgrep(before), pkill x3, pgrep(after), docker info, rm
+        # subprocess.run: pgrep(socket check, finds dockerd), pgrep(before kill),
+        # pkill x3, pgrep(after kill), docker info(dead), rm
         mock_run.side_effect = [
+            MagicMock(returncode=0, stdout="123 /usr/bin/dockerd\n"),  # socket check pgrep
             _pgrep_empty(),
             MagicMock(),
             MagicMock(),
@@ -218,6 +220,27 @@ class TestStartDockerDaemon:
         assert start_docker_daemon(timeout=5) is True
         mock_popen.assert_called_once()
 
+    @patch("devx.molecule.start_docker._diagnose_socket")
+    @patch("devx.molecule.start_docker._get_docker_free_bytes", return_value=0)
+    @patch("devx.molecule.start_docker.is_docker_ready", return_value=True)
+    @patch("devx.molecule.start_docker.os.path.exists", side_effect=_exists_map({DOCKER_SOCK}))
+    @patch("devx.molecule.start_docker.glob.glob", return_value=[])
+    @patch("devx.molecule.start_docker.subprocess.run")
+    def test_no_inner_dockerd_trusts_host_socket(
+        self,
+        mock_run: MagicMock,
+        mock_glob: MagicMock,
+        mock_exists: MagicMock,
+        mock_ready: MagicMock,
+        mock_free: MagicMock,
+        mock_diag: MagicMock,
+    ) -> None:
+        """Should trust /var/run/docker.sock with free=0 when no inner dockerd exists."""
+        # pgrep finds no dockerd processes (returncode=1)
+        mock_run.return_value = MagicMock(returncode=1, stdout="")
+        assert start_docker_daemon(timeout=5) is True
+        assert os.environ.get("DOCKER_HOST") == f"unix://{DOCKER_SOCK}"
+
     @patch("devx.molecule.start_docker._diagnose_socket")
     @patch("devx.molecule.start_docker._get_docker_free_bytes")
     @patch("devx.molecule.start_docker.is_docker_ready")