How to have an contained app systray icon on the host desktop manager

Hi,

I have an app that I can’t find with my current OS anymore to the version I want (nextcloud desktop client, for ubuntu 16.04).
So I built an image with the correct version of the app for another version of my OS.
Everything works well except the theming and the systray icon. Normally, the app, once run, would display a systray icon.
What option would be needed to a docker run to be able to have the systray icon activated?
Here is the way I run my image (Dockerfile):

docker run -d --name nextcloud -v /home/dolanor/var/nextcloud:/root/.local/share/data/Nextcloud -v /home/dolanor/cloud:/data -e DISPLAY=unix:$DISPLAY -v /etc/localtime:/etc/localtime:ro -v /tmp/.X11-unix:/tmp/.X11-unix nextcloud:2.3.2-utf8 nextcloud

Is it some package missing from the image, some X11 conf file that I need to pass? Some dbus dark magic?