Add support for cleaning up image while destroying a task

[Jadeiin]

The docker driver in Nomad provides the capability to clean up images when a task is destroyed (reference). This feature is essential for maintaining a clean and efficient environment, preventing the accumulation of stale or unused images which can consume significant disk space over time.

It would be highly beneficial if the podman driver in Nomad also supported this feature. Given that Podman is designed to be a drop-in replacement for Docker with a similar command structure, the ability to clean up images post-task destruction should ideally have a similar implementation.

[courtland]

FWIW we accomplish this with the following service override.

# /etc/systemd/system/podman-auto-update.service.d/override.conff
[Service]
ExecStartPost=
ExecStartPost=/usr/bin/podman image prune -a -f