DEVX-155: fix: use host Docker when root dir is inaccessible (free=0)

The CI container can't access the host Docker's data root
(/home/grm-ci-runner-X/.local/share/docker) to check disk space.
When free_bytes=0, the root dir is on the host filesystem (455G).
Always use host Docker in that case instead of starting a local
dockerd on the 16G /dev/shm tmpfs (which fills up with images).

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

tests/unit/test_start_docker.py

@@ -136,6 +136,16 @@ class TestStartDockerDaemon:
         mock_ready.assert_called_once()
         mock_diag.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)
+    def test_host_socket_inaccessible_root_uses_host(
+        self, mock_ready: MagicMock, mock_free: MagicMock, mock_diag: MagicMock
+    ) -> None:
+        """Should use host Docker when root dir is inaccessible (free=0)."""
+        assert start_docker_daemon(timeout=5) is True
+        mock_ready.assert_called_once()
+
     @patch("devx.molecule.start_docker._diagnose_socket")
     @patch("devx.molecule.start_docker._get_docker_free_bytes", return_value=5 * 1024**3)
     @patch("devx.molecule.start_docker.is_docker_ready", return_value=True)