[Request]: Support a mirror config for the vminit image

### Feature or enhancement request details

So was trying to figure out the sharp edges in our environment and didn't take long.

Image pulls were working fine from our private registry, but a run was failing `"Missing required header Docker-Content-Digest"` which i was attributing to something funny w our registry. Now image digest was looking fine so was scratching my head a bit.

Took a look at the `system logs` though and figured it out
```
2025-09-22 17:15:20.983133-0400 0x19cfcd Info 0x0 6795 0 container-core-images: [com.apple.container:ImagesHelper] ImagesService: pull(reference:platform:insecure:progressUpdate:) - ref: ghcr.io/apple/containerization/vminit:0.6.2, platform: Optional(ContainerizationOCI.Platform(osVersion: nil, osFeatures: nil, variant: Optional("v8"), _rawOS: "linux", _rawArch: "arm64")), insecure: false
2025-09-22 17:15:21.603920-0400 0x19cfce Error 0x0 6795 0 container-core-images: [com.apple.container:ImagesHelper] handler for imagePull threw error invalidArgument: "Missing required header Docker-Content-Digest"
```
We don't have a clear shot to ghcr.io which is what was actually causing the error. I tried pulling the vminit local via our private registry, but no luck. Also tried `container registry default` to the private but the logic seems to be compiled in.

Wasn't sure if there was another way to manually do an init so i can figure out the next set of challenges?

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