mz334: move resolveCrossStageCommands forwards

[mzihlmann]

This is a refactoring in preparation for #334

Description

When we create KanikoStage's we replace all baseImage references with numeric references. But we keep both named and numeric references in COPY --from. ie COPY --from=base or COPY --from=0 (yes this is valid syntax). Later we replace those references a bit hidden as part of ResolveCrossStageInstructions. With this change we just move that transformation to the beginning, so when you have a KanikoStage you are guaranteed to only have numeric references to other stages.

[mzihlmann]

note that this temporarily makes the whole ONBUILD situation worse #332
Maybe we should fix that first as a priority before taking this refactoring. On the other hand I don't expect a single person to actually run into this bug as it appears a pretty useless feature in the spec to me.

Well no, currently you can workaround that issue by not taking any optimizations, ie. --skip-unused-stages=false. If we do this refactoring that is no longer possible and it starts to break for them too. So let's push this out a bit.

[Copilot]

Pull Request Overview

This PR refactors the handling of cross-stage command resolution by moving the transformation of named stage references to numeric indices earlier in the build process. Instead of resolving COPY --from=<stage_name> to COPY --from=<index> later during ResolveCrossStageInstructions, this transformation now happens immediately when KanikoStage objects are created in MakeKanikoStages.

Key changes:

• Cross-stage command resolution moved from ResolveCrossStageInstructions to MakeKanikoStages
• Simplified fetchExtraStages to only handle numeric stage references and remote images
• Updated skipUnusedStages to remove named reference handling logic

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
pkg/executor/build.go	Removed named stage tracking in fetchExtraStages and call to ResolveCrossStageCommands in ResolveCrossStageInstructions
pkg/dockerfile/dockerfile.go	Made ResolveCrossStageCommands private, added stage name resolution in MakeKanikoStages, simplified skipUnusedStages
pkg/dockerfile/dockerfile_test.go	Updated tests to use MakeKanikoStages and set feature flag environment variable

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[mzihlmann]

as the ONBUILD references are now resolved earlier, we can safely take this refactoring

[mzihlmann]

AI summary review:

Overview

Refactoring that moves cross-stage reference resolution (e.g., COPY --from=base → COPY --from=0) from ResolveCrossStageInstructions to earlier in MakeKanikoStages. This guarantees that KanikoStage objects only contain numeric stage references.

Key Changes

1. Made function private: ResolveCrossStageCommands → resolveCrossStageCommands
2. Early resolution: Called during MakeKanikoStages instead of later in pipeline
3. Simplified fetchExtraStages: Removed stage name tracking logic since all refs are now numeric
4. Removed fromPreviousStage helper: No longer needed
5. Cleanup: Removed duplicate error checks in tests

Analysis

✅ Strengths

• Better architecture: Centralizes transformation logic and makes guarantees explicit
• Code simplification: Removes ~30 lines of duplicate logic for tracking named stages
• Improved validation: fetchExtraStages now properly validates stage indices
• Clean refactoring: Well-structured with appropriate visibility changes

🔍 Considerations

• Test coverage: Verify existing tests cover both numeric and named COPY --from syntax
• Backward compatibility: Confirm this doesn't break any edge cases with named stage references
• Error messages: The new error in fetchExtraStages is clear and helpful

📝 Minor Notes

• Good cleanup of duplicate testutil.CheckError calls
• Function visibility change (public→private) is appropriate for internal refactoring

Recommendation

✅ APPROVE - Solid refactoring that simplifies code and improves architecture. No significant risks identified.