cg160: ocilayout warmer

[mzihlmann]

Fixes # in case of a bug fix, this should point to a bug and any other related issue(s)

Description

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

[mzihlmann]

good now, but lacks integration tests

[mzihlmann]

currently builds on top of #321, let's get the other in first.

[mzihlmann]

integration tests dont work locally anymore after activating this, simply because a subdir in integration/cache and integration/tmpCache gets created that can't be walked by go test.

[Copilot]

Pull Request Overview

This PR enhances the Kaniko image caching mechanism to properly handle OCI image layouts and digest-based cache lookups. It adds a new OCI warmer feature behind a feature flag that stores cached images in OCI layout format rather than tarballs, and fixes cache lookup logic to correctly handle the distinction between image manifests and image indexes.

Key changes:

• Improved cache lookup logic to handle digest references that may point to either image manifests or image indexes
• Added OCI-based warmer implementation behind FF_KANIKO_OCI_WARMER feature flag
• Added integration tests for the new warmer functionality

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/image/image_util.go	Enhanced cache lookup to attempt local cache first for digest references, with fallback to remote lookup for image-index cases
pkg/cache/warm.go	Added OciWarmer struct and ociWarmToFile function to support OCI layout caching
pkg/cache/cache.go	Added ociCachedImageFromPath to read images from OCI layout directories
integration/integration_test.go	Added TestWarmer and manifestDiff tests, refactored helper functions
integration/images.go	Added warmer test infrastructure, updated base image references, added buildWarmerImage method
integration/dockerfiles/Dockerfile_test_issue_mz320	New test Dockerfile for testing image-manifest vs image-index cache behavior

[0hlov3]

LGTM.

[mzihlmann]

just noticed this misses the readme docs for FF_KANIKO_OCI_WARMER