Looking for way to create virtual gui desktop container inside container e.g docker or LXD whichever is better
while not really made for it something that would appear like a classic VM (as outlined by @xenoid) you can achieve "a full remote desktop in the container" via xrdp, see this reddit post for some details.
The TL;DR would be:
$ sudo apt install xrdp $ sudo apt install xorgxrdp $ sudo apt install xrdp-pulseaudio-installer
I think depending on the ubuntu version you might need to enable the services and then connect with the RDP client of your choice.