Kanboard behind traefik with 'path'

I do succeed to run kanboard in docker as a single webapp on an URL,
but now, I need to make it works behind a traefik which will deploy many apps.

As I can’t play with sub-domains like http://kanboard.server.lan
I need to map it on http://server.lan/kanboard

I’ve tried things with docker-compose, the most working thing I get is a 302 redirect " location /login"
How to tell kanboard his “root” is /kanboard ?

Failed to use "define(‘KANBOARD_URL’…) in config.php
Failed to use the setting in admin > settings > app url