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).

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