How to add br_netfilter to an LXD container

You need to create or use an existing profile that enables the br_netfilter module

$ lxc profile create mod_br_netfilter
$ lxc profile set mod_br_netfilter linux.kernel_modules br_netfilter
$ lxc profile show mod_br_netfilter
  name: mod_br_netfilter  
  config:
    linux.kernel_modules: br_netfilter
  description: ""
  devices: {}
$ lxc launch ubuntu:latest YOURcontainer -p default -p mod_br_netfilter
############ To apply to an existing Instance #####################
$ lxc profile apply YOURcontainer default, mod_br_netfilter
$ lxc restart YOURcontainer

Alternatively you can edit any profile you already use to support this kernel module

$ lxc profile edit default

edit the config section to add the configuration key linux.kernel_modules: br_netfilter