Canonical's LXD and its community fork Incus: system containers plus real VMs under one excellent CLI, a REST API, and a built-in web UI. Minimal overhead, clustering that automatically lands new instances on the least-loaded member, container density plus full virtual machines when you need them. The fork politics, the auto-placement scriptlet, the lighter multi-tenancy story, and why this is the best low-effort path to self-service-with-placement for mostly-Linux workloads.
Incus
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Incus and LXD: The Lightweight Cluster That Auto-Places