libimage: clarify local image transport error

[Rajkaran-122]

Summary

Clarify the error returned by Runtime.LookupImage when a local image lookup is attempted using a non-storage transport such as docker://.

Previously the error identified the unsupported transport but did not explain how to refer to a locally stored image. This change preserves the existing rejection while adding guidance to remove the transport prefix.

Changes

• Clarify the error returned for non-storage transports in Runtime.LookupImage
• Extend TestLookupImage to cover docker:// references
• Preserve existing lookup and error propagation behavior

Testing

go test -run '^TestLookupImage$' -count=1 -tags containers_image_openpgp ./libimage

The targeted test exceeded the local execution limit because the existing test pulls Alpine.

Also verified:

git diff --check

[github-actions]

This repository has been migrated to https://github.com/containers/container-libs. Please open your PR there.