Proxmox - Guests (VMs and Containers) - Install a GUI - A more functional minimal desktop environment

sudo apt install xorg
sudo apt install --no-install-recommends lightdm-gtk-greeter
sudo apt install --no-install-recommends lightdm
sudo apt install --no-install-recommends lxde-icon-theme
sudo apt install --no-install-recommends lxde-core
sudo apt install --no-install-recommends lxde-common
sudo apt install --no-install-recommends policykit-1 lxpolkit
sudo apt install --no-install-recommends lxsession-logout
sudo apt install --no-install-recommends gvfs-backends

NOTE:

lxde-icon-theme is needed for basic icons (there are alternatives).
lxde-core and lxde-common will install the basic lxde components.
policykit-1 and lxpolkit are needed to run pkexec.
lxsession-logout is needed so that the logout menu works.
gvfs-backends is needed if you want trash, network, devices etc. support at pcmanfm.