Tailscale in docker: use exit node with mullvad

I am using tailscale in combination with the mullvad addon.

I would like to make one of my dockers use a mullvad exit node by default.

When choosing an exit node in the settings for the docker I can only choose my own devices that I made an exit node, not the mullvad addons.

Is that a feature to come or a bug ?

I also noticed that the "tailscale webui" does not take the port into account. It takes you to the tailscale IP but does not use the port, therefore for most dockers it will not work.