fix(builtin): bring kernel bridge UP after creation

brctl create leaves the bridge administratively DOWN. Add link set up
so the bridge actually forwards frames between enslaved ports.

Also record Docker iptables FORWARD DROP pitfall in project memory.

gns3server/compute/builtin/nodes/ethernet_switch.py

@@ -227,6 +227,8 @@ class EthernetSwitch(BaseNode):
             return
         self._bridge_name = self._free_iface("gns3br")
         await self._ubridge_send(f'brctl create "{self._bridge_name}"')
+        # ``brctl create`` leaves the bridge DOWN; bring it UP so it forwards.
+        await self._ubridge_send(f'link set "{self._bridge_name}" up')
         await self._ubridge_send(f'brctl vlanfiltering "{self._bridge_name}" on')
         self._bridge_created = True
         await self._apply_bridge_proto_if_needed()