Proxmox - Guests (VMs and Containers) - Install a GUI - A full lightweight desktop environment

sudo apt install xorg

Then choose one of these: 

sudo apt install --no-install-recommends lubuntu-core

OR 

sudo apt install --no-install-recommends xubuntu-core

OR 

sudo apt install --no-install-recommends ubuntu-mate-core

NOTE: Each of these metapackages is based on lxde.

  • xfce and mate desktop respectively including dependencies such as alsa, lightdm etc. and with many more packages such as themes, configurations etc.