This is purely cosmetic - if a non-default MTU is configured, the bridge
will have the default MTU=1500 until a container's 'veth' is connected
and an MTU is set on the veth. That's a disconcerting, it looks like the
config has been ignored - so, set the bridge's MTU explicitly.
Fixes#37937
Signed-off-by: Rob Murray <rob.murray@docker.com>