That link you referred to is the mini.iso bootable image for Ubuntu which only includes the text installer (debian-installer) and isn’t a working Ubuntu system at all.
It doesn’t make any sense to run this inside a container as that image is meant to install a full Ubuntu machine (host) and isn’t a usable system by itself.
But it is a usable Ubuntu for me! It just defaults to a bare minimum needed. I installed it on a VM and just installed the Ubuntu Server and SSH. I have only one use for the container, so I only need the app I will install, nginx and SSH. Make sense?