Automatic Separation of /tmp for LXD Containers

[Aymen Frikha]

Currently, Juju does not support an automated process for separating the /tmp directory for LXD containers onto a separate disk partition. Administrators must manually intervene using LXD-specific procedures, which is not feasible or scalable for large deployments.

This is how we do it with lxd:
```
lxc storage volume create default c1-tmp size=1GiB
lxc config device add c1 tmp disk source=c1-tmp pool=default
lxc start c1
lxc exec c1 -- findmnt --mountpoint /tmp
TARGET SOURCE FSTYPE OPTIONS
/tmp /dev/mapper/vgubuntu-root[/var/lib/lxd/storage-pools/default/custom/default_c1-tmp] ext4 rw,relatime,errors=remount-ro
```

The lack of this feature complicates compliance with CIS standards, potentially exposing deployments to security risks associated with shared /tmp directories.

[Aymen Frikha (aym-frikha)]

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