[FEATURE] Implement image index for multi-arch images

[adambkaplan]

Overview

This task implements the logic to create an image index containing each architecture-specific image digest, as described in SHIP-0043.

Description

After all architecture-specific builds complete successfully, create an OCI image index that references the individual architecture-specific image digests:

• Collecting image references from all architecture-specific builds
• Creating a manifest list (OCI Image Index) that references all architecture-specific images
• Pushing the image index to the registry
• Handling errors if manifest index creation fails
• Handling cases where some architectures fail (fail the build)

Acceptance Criteria

• Logic to collect image references from all architecture builds
• Multi-arch manifest list (OCI Image Index) creation
• Manifest list push to registry
• Error handling for manifest index creation failures
• Handling of partial failures (some architectures succeed, others fail - should fail the build)
• Unit tests pass with >80% coverage for new code
• Integration tests verify image index creation completes successfully
• E2E tests for complete multi-arch build workflow including image index creation

Dependencies

• Blocked by: Implement multi-arch build API, orchestration, and node selection
• Blocks: Documentation and examples

Technical Notes

• Use container image libraries (e.g., github.com/containers/image) for manifest operations
• Manifest list should follow OCI Image Index specification
• Consider using a final task in the PipelineRun to create the image index
• Handle cases where some architectures fail (this should fail the build)
• Image index creation should happen after all successful architecture builds complete
• Ensure manifest list includes proper architecture annotations for each image

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Generated-by: Cursor

[adambkaplan]

Related to shipwright-io/community#285 - SHIP-0043 Multi-arch Image Builds