As the lecture says we can’t use docker to run an operating system
so, what does run a “ubuntu” means?
isn’t ubuntu an Operating system where "Linux " is a kernel.
Yes Ubuntu is an operating system and as part of installing it on bare metal or a VM it will run under a Linux Kernel. With containers you run an application or operating system within Docker with some separation and abstraction from the host operating system but it shares the Kernel and hardware.
So is possible to have a Centos OS and run an Ubuntu container on it.