DEVX-155: fix: kill existing dockerd before starting /dev/shm/docker daemon

The CI container's pre-existing dockerd (v29.5.3) uses the default
data root on the 38G overlay (100% full). Kill it before starting
the new dockerd with --data-root /dev/shm/docker (16G tmpfs).

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

@@ -146,8 +146,12 @@ class TestStartDockerDaemon:
         with (
             patch("devx.molecule.start_docker.glob.glob", return_value=[]),
             patch("devx.molecule.start_docker.os.path.exists", return_value=False),
+            patch("devx.molecule.start_docker.subprocess.run"),
+            patch("devx.molecule.start_docker.subprocess.Popen"),
+            patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile"),
+            patch("devx.molecule.start_docker.time.sleep"),
         ):
-            # Host ready but low space, no rootless sockets, falls back to host
+            # Host ready but low space, no rootless sockets, starts local dockerd
             assert start_docker_daemon(timeout=5) is True
 
     @patch("devx.molecule.start_docker._diagnose_socket")
@@ -202,12 +206,14 @@ class TestStartDockerDaemon:
     @patch("devx.molecule.start_docker.os.path.exists", return_value=False)
     @patch("devx.molecule.start_docker.is_docker_ready", return_value=False)
     @patch("devx.molecule.start_docker.time.sleep")
+    @patch("devx.molecule.start_docker.subprocess.run")
     @patch("devx.molecule.start_docker.subprocess.Popen")
     @patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile")
     def test_starts_local_daemon(
         self,
         mock_ntf: MagicMock,
         mock_popen: MagicMock,
+        mock_run: MagicMock,
         mock_sleep: MagicMock,
         mock_ready: MagicMock,
         mock_exists: MagicMock,
@@ -232,12 +238,14 @@ class TestStartDockerDaemon:
     @patch("devx.molecule.start_docker.os.path.exists", return_value=False)
     @patch("devx.molecule.start_docker.is_docker_ready", return_value=False)
     @patch("devx.molecule.start_docker.time.sleep")
+    @patch("devx.molecule.start_docker.subprocess.run")
     @patch("devx.molecule.start_docker.subprocess.Popen")
     @patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile")
     def test_fails_after_timeout(
         self,
         mock_ntf: MagicMock,
         mock_popen: MagicMock,
+        mock_run: MagicMock,
         mock_sleep: MagicMock,
         mock_ready: MagicMock,
         mock_exists: MagicMock,
@@ -249,7 +257,7 @@ class TestStartDockerDaemon:
         with patch("builtins.open", mock_open(read_data="dockerd error log")):
             assert start_docker_daemon(timeout=3) is False
         mock_popen.assert_called_once()
-        assert mock_sleep.call_count == 3
+        assert mock_sleep.call_count == 4  # 1 pkill + 3 timeout
 
     @patch("devx.molecule.start_docker.glob.glob", return_value=[])
     @patch("devx.molecule.start_docker._diagnose_socket")
@@ -257,12 +265,14 @@ class TestStartDockerDaemon:
     @patch("devx.molecule.start_docker.os.path.exists", return_value=False)
     @patch("devx.molecule.start_docker.is_docker_ready", return_value=False)
     @patch("devx.molecule.start_docker.time.sleep")
+    @patch("devx.molecule.start_docker.subprocess.run")
     @patch("devx.molecule.start_docker.subprocess.Popen")
     @patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile")
     def test_fails_log_read_error(
         self,
         mock_ntf: MagicMock,
         mock_popen: MagicMock,
+        mock_run: MagicMock,
         mock_sleep: MagicMock,
         mock_ready: MagicMock,
         mock_exists: MagicMock,
@@ -281,12 +291,14 @@ class TestStartDockerDaemon:
     @patch("devx.molecule.start_docker.os.path.exists", return_value=False)
     @patch("devx.molecule.start_docker.is_docker_ready")
     @patch("devx.molecule.start_docker.time.sleep")
+    @patch("devx.molecule.start_docker.subprocess.run")
     @patch("devx.molecule.start_docker.subprocess.Popen")
     @patch("devx.molecule.start_docker.tempfile.NamedTemporaryFile")
     def test_local_daemon_ready_on_first_check(
         self,
         mock_ntf: MagicMock,
         mock_popen: MagicMock,
+        mock_run: MagicMock,
         mock_sleep: MagicMock,
         mock_ready: MagicMock,
         mock_exists: MagicMock,
@@ -299,7 +311,7 @@ class TestStartDockerDaemon:
         mock_ready.side_effect = [False, False, True]
         assert start_docker_daemon(timeout=5) is True
         assert mock_popen.call_count == 1
-        assert mock_sleep.call_count == 1
+        assert mock_sleep.call_count == 2  # 1 pkill + 1 timeout
 
     @patch("devx.molecule.start_docker._diagnose_socket")
     @patch("devx.molecule.start_docker._get_docker_free_bytes", return_value=100 * 1024**3)