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.

tests/api/routes/compute/test_ethernet_switch_nodes.py

@@ -85,6 +85,7 @@ class TestEthernetSwitchNodesRoutes:
         br = node._bridge_name
         node._ubridge_send.assert_has_calls([
             call(f'brctl create "{br}"'),
+            call(f'link set "{br}" up'),
             call(f'brctl vlanfiltering "{br}" on'),
         ])