Create a container with a dedicated (non-shared) disk volume — storage isolation from other boxes

[hsinhoyeh]

Summary

Let a container be created with its own dedicated disk volume, isolated from
other containers — instead of every box carving its disk quota out of the one
shared host storage pool. A box that needs guaranteed, non-contended storage
(a database, a build cache, a data-heavy workload) should be able to sit on a
disk that isn't shared with, or starved by, its neighbours.

Today

Boxes get a disk quota (--disk 50GB), but that quota is a limit within the
shared backend storage pool (default, driver dir/zfs). All boxes on a
host draw from the same pool: they share the underlying device's IOPS and free
space, and a noisy or runaway neighbour can contend for both. There's no way to
say "give this box its own volume/disk."

Ask

A create-time option to place a container's disk on a dedicated volume the
box doesn't share:

• CLI/proto: e.g. containarium create <box> --dedicated-disk[=<size>]
  (and the matching create-request field + MCP create_container arg), so the
  choice is first-class and proto-first like the rest of the create surface.
• Backend: provision an isolated volume for the box rather than a quota in
  the shared pool. Options to weigh in design:
  • a dedicated Incus storage volume (or a per-box pool) so space + IOPS are
    the box's own;
  • a loopback/dataset-backed volume attached as the box's disk;
  • on the K8s runtime, a dedicated PVC / StorageClass (partly there already
    via storage_class, but not "isolated from other boxes" as a contract).
• Isolation contract: the box's disk usage can't be starved by neighbours,
  and (ideally) its IO is not co-mingled on the same device where the
  deployment provides separate devices.

Why

• Noisy-neighbour isolation for storage (we already isolate CPU/mem and
  network; disk is the gap).
• Predictable capacity — a data box's space isn't eaten by unrelated boxes
  filling the shared pool (cf. the disk-full incidents we've hit).
• Cleaner teardown / portability of a box's data volume.

Notes

• Relates to the existing storage_class field (K8s) — that selects a class but
  doesn't guarantee non-shared isolation on the LXC backend.
• Design should decide the LXC mechanism (dedicated Incus volume vs per-box
  pool vs dataset) and whether size is required or optional (default = today's
  shared-pool quota when the flag is absent, so this is purely additive).