X11 forwarding of a GUI app running in docker

Thanks so much @Lazarus535
I found that for me adding the following to my docker command worked:
--volume="$HOME/.Xauthority:/root/.Xauthority:rw"
I found this trick here
EDIT:
As Lazarus pointed out correctly you also have to set the --net=host option to make this work.

Ok, here is the thing:

1) Log in to remote machine

2) Check which display was set with echo $DISPLAY

3) Run xauth list

4) Copy the line corresponding to your DISPLAY

5) Enter your docker container

6) xauth add <the line you copied>*

7) Set DISPLAY with export DISPLAY=<ip-to-host>:<no-of-display>

*so far so good right?

This was nothing new...however here is the twist: The line printed by xauth list for the login user looks something like this (in my case):

<hostname-of-machine>/unix:<no-of-display> MIT-MAGIC-COOKIE-1 <some number here>

Because i use the bridged docker setup, the X forwarding port is not listening locally, because the sshd is not running in the container. Change the line above to:

<ip-of-host>:<no-of-display> MIT-MAGIC-COOKIE-1 <some number here>

In essence: Remove the /unix part.

<ip-of-host> is the IP address where the sshd is running.

Set the DISPLAY variable as above.

So the error was that the DISPLAY name in the environment variable was not the "same" as the entry in the xauth list / .Xauthority file and the client could therefor not authenticate properly.

I switched back to an untrusted X11 forwarding setting.

The X11UseLocalhost no setting in the sshd_config file however is important, because the incomming connection will come from a "different" machine (the docker container).