feat: make the docker binary path a hook

Where docker lives is device-specific; what to do with it is not. QNAP's
Container Station installs docker under /share/<pool>/.qpkg/ and never
puts it on PATH, so a QTS driver inheriting this class found no docker at
all.

Rather than reimplementing the Docker surface in the vendor driver, the
path becomes a single overridable method and every call site goes through
it. Per docs/ARCHITECTURE.md 4.4, generic logic belongs to the shared
driver and only the device-specific mechanics belong to the vendor one.

A test asserts that *every* docker call site uses the hook — a half
converted set would let detection find the binary while the actual
queries still missed it, which only shows up against real hardware.

tests/test_linux.py

@@ -802,3 +802,54 @@ class TestRunDeviceActionDispatch:
         ) as mock_action:
             driver.run_device_action("apt_update_upgrade")
         mock_action.assert_called_once()
+
+
+class TestDockerBinHook:
+    """Where the docker binary lives is device-specific; what to do with it is not.
+
+    QTS puts Container Station's docker under /share/<pool>/.qpkg/ and not on
+    PATH. Rather than duplicating the whole Docker surface in the QNAP driver,
+    the path is a one-method hook here and the logic stays generic. See
+    docs/ARCHITECTURE.md §4.4, "Generisch vs. treiberspezifisch".
+    """
+
+    def test_defaults_to_docker_on_path(self, driver):
+        assert driver._docker_bin() == "docker"
+
+    def test_detection_uses_the_hook(self, driver):
+        """A subclass pointing elsewhere must not be probed for a PATH docker."""
+        sent = []
+
+        def _record(cmd, **kwargs):
+            sent.append(cmd)
+            return ""
+
+        with patch.object(driver, "_docker_bin", return_value="/opt/cs/docker"):
+            with patch.object(driver, "_send", side_effect=_record):
+                result = driver.get_docker_info()
+
+        assert result == {"available": False}
+        assert any("/opt/cs/docker" in cmd for cmd in sent)
+        assert not any("command -v docker " in cmd for cmd in sent)
+
+    def test_all_docker_subcommands_use_the_hook(self, driver):
+        """Half-converted call sites are the failure mode here: detection would
+        find the binary and the actual queries would still miss it."""
+        sent = []
+
+        def _record(cmd, **kwargs):
+            sent.append(cmd)
+            if "command -v" in cmd:
+                return "/opt/cs/docker"
+            if "---CONTAINERS---" in cmd:
+                return "---CONTAINERS---\n---IMAGES---\n---VOLUMES---\n---NETWORKS---\n"
+            return ""
+
+        with patch.object(driver, "_docker_bin", return_value="/opt/cs/docker"):
+            with patch.object(driver, "_send", side_effect=_record):
+                driver.get_docker_info()
+
+        docker_cmds = [c for c in sent if "docker" in c]
+        assert docker_cmds
+        for cmd in docker_cmds:
+            assert "/opt/cs/docker" in cmd, f"unconverted call site: {cmd}"