libimage: clarify local image transport error

[Rajkaran-122]

Summary

Runtime.LookupImage rejects non-storage transport references for local image lookups. The previous error identified the unsupported transport but did not explain how to refer to a locally stored image.

Clarify that the lookup operates on local storage and instruct users to remove the transport prefix while preserving the existing rejection behavior.

Changes

• improve the Runtime.LookupImage error message for unsupported transports
• extend TestLookupImage to cover docker:// references
• preserve existing lookup and error propagation behavior

Testing

• added a lookup test for docker:// references
• ran gofmt
• ran git diff --check

The focused lookup test was attempted locally but exceeded the execution limit because the existing test pulls Alpine.

[Rajkaran-122]

It looks like the Packit job is failing before the build starts due to invalid chroot parameters rather than a build or test failure. Please let me know if there's anything I should update in this PR, or if this is a CI configuration issue.

[mtrmac]

Thanks.

The user explicitly asked for a registry image. Nothing suggests the user wants to refer to a local image. In such a situation, sure, the user is probably confused either about what is the operation being attempted, or about what the inputs mean.

But why is it helpful to tell the user how to refer to a local image? That might allow the user to “proceed” without actually clearing up any misunderstandings — potentially resulting in more damage when the user does not understand what is going to happen.

A reminder: https://github.com/podman-container-tools/podman/blob/main/LLM_POLICY.md .

[Rajkaran-122]

Thanks for the feedback @mtrmac sir . I updated the error message to remove the suggestion about removing the transport prefix. It now simply clarifies that LookupImage only supports local-storage image references, without assuming the user's intent or suggesting an alternative operation. The behavior is otherwise unchanged, and I updated the existing test to match the revised message.