docker: also null-bind udevadm in GNS3_MASK_UDEV

Masking the udev systemd units stopped the daemon's coldplug (host audio
resets), but host USB devices still reconnected on every XRd start. A/B
testing with plain `docker run` isolated the trigger: XRd's own
xr_startup.sh calls udevadm directly (USB license-dongle probing, e.g.
`udevadm trigger --action=add --parent-match=<usb device>`), which
synthesizes uevents into the host kernel from the privileged container --
no udevd required.

GNS3_MASK_UDEV=1 now also binds /dev/null over the udevadm binary
(/bin, /sbin, /usr/bin). Verified with a plain-run experiment: with the
bind, host udev monitor shows zero usb/input/hid/sound events during XRd
boot (only normal docker veth traffic), and XRd itself boots to running
state -- it does not need udevadm under GNS3 (interfaces are pre-created
veths).

gns3server/compute/docker/docker_vm.py

@@ -80,6 +80,19 @@ class DockerVM(BaseNode):
         "systemd-udev-settle.service",
     )
 
+    # udevadm binary paths also null-bound by GNS3_MASK_UDEV=1. NOS startup
+    # scripts call udevadm directly -- Cisco XRd's xr_startup.sh runs
+    # `udevadm trigger --action=add --parent-match=<usb device>` (USB license
+    # dongle probing), which synthesizes uevents into the host kernel from a
+    # privileged container and reconnects host USB devices. Masking the units
+    # alone does not stop this; the binary must be neutralized too. XRd boots
+    # fine without udevadm (interfaces are pre-created by GNS3).
+    _UDEVADM_PATHS = (
+        "/bin/udevadm",
+        "/sbin/udevadm",
+        "/usr/bin/udevadm",
+    )
+
     def __init__(
         self,
         name,
@@ -561,11 +574,11 @@ class DockerVM(BaseNode):
                     # audio/disk devices, reconnecting/muting them on every start.
                     # XRd doesn't need udev (interfaces are pre-created by GNS3), so
                     # bind /dev/null over the udev units to keep it from running.
-                    for unit in self._UDEV_UNITS:
+                    for target in [f"/etc/systemd/system/{u}" for u in self._UDEV_UNITS] + list(self._UDEVADM_PATHS):
                         params["HostConfig"]["Mounts"].append({
                             "Type": "bind",
                             "Source": "/dev/null",
-                            "Target": f"/etc/systemd/system/{unit}",
+                            "Target": target,
                             "ReadOnly": True,
                         })
                 elif line.startswith("GNS3_MASK_SYSTEMD="):

tests/compute/docker/test_docker_vm.py

@@ -327,6 +327,8 @@ async def test_create_masks_systemd_units(compute_project, manager):
                       if m.get("Source") == "/dev/null"}
             for unit in DockerVM._UDEV_UNITS:
                 assert f"/etc/systemd/system/{unit}" in masked
+            for path in DockerVM._UDEVADM_PATHS:
+                assert path in masked
             assert "/etc/systemd/system/foo.service" in masked
             assert "/etc/systemd/system/bar.socket" in masked