container

One process on your server, started by the same kernel as every other, except that the kernel shows it a restricted view: its own filesystem tree, its own process table, its own network interfaces. ps aux on the host lists it among the rest; ps aux inside the container sees only itself, and that narrow view is the whole illusion of a machine. No second kernel, no hypervisor: a container is therefore not the security boundary a virtual machine is. A kernel flaw reached from inside is reached on the host, root in the container is still UID 0 talking to your kernel until you decide otherwise, and a published port lands in netfilter rather than in ufw's rules. Code you would not run on the server, you are running on the server.