fix(docker): bring the veth host end up before a relay attach

The unified veth host end is born admin-down (carrier off until a link
attaches), and libpcap cannot open a packet socket on an admin-down
interface — its netlink promiscuous-mode transaction returns ENOENT, so
'bridge add_nio_ethernet' failed on every runtime relay link (and would
on any relay link at container start). Bring the interface up for the
attach; the carrier pass in the caller refines the state afterwards (a
suspended NIO sets it back down). Verified live with a direct uBridge
probe: attach on a down veth -> ENOENT, after 'link set up' -> OK.

tests/compute/docker/test_docker_kernel_datapath.py

@@ -222,6 +222,9 @@ async def test_add_ubridge_connection_udp_relays_over_veth(vm):
         [
             call.send(f'docker create_veth "{host_ifc}" "{vm._veth_names(0, 0)[1]}"'),
             call.send("bridge create bridge0"),
+            # the host end is born down; libpcap cannot attach to an
+            # admin-down interface, so the relay brings it up first
+            call.send(f'link set "{host_ifc}" up'),
             call.send(f'bridge add_nio_ethernet bridge0 "{host_ifc}"'),
             call.send("bridge add_nio_udp bridge0 4242 127.0.0.1 4343"),
             call.send("bridge start bridge0"),