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.

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• 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]

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

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