LXD, its more recent variant, spearheaded by Canonical) is the oldest Linux container virtualization technology, predating Docker and providing a VM-like abstraction atop a shared kernel.
I’ve been using it for many, many years, and find it a great way to run sandboxes and test out VM instance automation without the overhead of
KVM or a comparable hypervisor.
a Debian-based hypervisor with great LXC support
a tool to manage LXC UID mappings inside Proxmox
an LXD front-end from Canonical
An LXD dashboard