Support air-gapped container creation using locally cached OCI images

[ArshSSandhu]

Summary

Built-in container templates still require registry access even when the required OCI image has already been transferred to the Proxmox host.

This prevents normal container creation through Manager in an air-gapped environment.

Steps to Reproduce

1. Run Manager on a Proxmox node with no Internet access.
2. In Manager, create a new container and select the built-in Debian 13 template.
3. Manager attempts to retrieve image metadata from GHCR and eventually times out.
4. Download ghcr.io/mieweb/opensource-server/base:latest on an Internet-connected machine and transfer the OCI archive to Proxmox.
5. Retry container creation through Manager.

Actual Behavior

Manager still attempts to contact the registry before using the locally available image, so container creation fails when registry access is unavailable.

The OCI archive itself can be used successfully by Proxmox when created manually, which confirms the image is available and usable offline.

Expected Behavior

If the required OCI image is already available in Proxmox storage, Manager should be able to create the workload without requiring registry access.

Proposed Direction

Allow Manager to check for and use a locally cached OCI image before requiring registry metadata, or provide a supported way to import/cache images for air-gapped deployments.

[ArshSSandhu]

@runleveldev I created this issue from the air-gapped cluster testing we discussed. Assigning is disabled for me, so could you review it and assign it back to me if you'd like me to work on it?