feat(driver-podman): add userns config

[giuseppe]

Add a userns option to the Podman compute driver that maps to Podman's --userns flag. When set to auto, the container spec includes idmappings.AutoUserNs = true as required by the API.

Podman image volumes use overlay mounts internally and the kernel does not support idmapped mounts on overlay. When userns is configured, the driver extracts the supervisor binary from the image to a host-side cache and bind-mounts it instead of using an image volume. The image volume path is preserved when userns is unset.

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