feat: Implement multi-arch build orchestration and image index assembly

[anchi205]

Changes

Generate per-platform PipelineTasks with architecture-specific nodeSelectors so each build runs on a matching node. Source code is bundled as an OCI artifact during source-acquisition and pulled on each platform node. Per-platform tasks suffix the output image tag with the platform to prevent registry GC from deleting intermediate images.

After all platform builds complete, an assemble-index task creates an OCI image index referencing each platform image. Result aggregation populates BuildRunStatus.PlatformResults with per-platform digest, size, vulnerabilities, and status, and sets ManifestDigest and Output.Digest to the index digest. Partial failure is tracked per platform so users can identify which architecture failed.

CLI flags --push-source-bundle and --assemble-index are added to the image-processing binary, consumed by the source-acquisition and assemble-index pipeline tasks respectively.

Related Issues

Fixes #2143
Fixes #2076

Type of PR

/kind feature

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Kind label has been set
• Release notes block has been filled in, or marked NONE

Release Notes

Multi-architecture container image builds. When spec.output.platforms is set, the controller runs parallel per-platform builds on architecture-matched nodes and assembles an OCI image index. Per-platform status, digest, and vulnerabilities are reported in status.platformResults.

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[Copilot]

Pull request overview

This PR adds first-class multi-architecture build support to the BuildRun PipelineRun executor by fanning out per-platform build tasks (scheduled onto matching nodes), pushing/pulling source as an OCI artifact, and assembling a final OCI image index. It also extends status reporting to include per-platform results and updates docs/CRDs/tests accordingly.

Changes:

• Generate multi-arch PipelineRuns: per-platform build-<os>-<arch> tasks + assemble-index, with per-task OS/arch nodeSelectors and EmptyDir workspaces.
• Add image-processing CLI support for --push-source-bundle and --assemble-index, plus an AssembleImageIndex helper.
• Report per-platform digests/sizes/vulnerabilities in .status.platformResults and set .status.output.digest to the index digest; add unit + e2e coverage and update docs/CRDs.

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/utils/v1beta1/taskruns.go	Add helper to list/sort TaskRuns for a PipelineRun by label for e2e assertions.
test/utils/v1beta1/image.go	Fix digest validation for multi-platform outputs by using remote.Get (index-safe).
test/e2e/v1beta1/e2e_pipelinerun_test.go	Add e2e coverage for multi-arch spec.output.platforms and index assembly behavior.
test/e2e/v1beta1/common_suite_test.go	Extend test build prototype to set spec.output.platforms.
pkg/reconciler/buildrun/resources/results.go	Populate .status.platformResults and set output digest from assemble-index results.
pkg/reconciler/buildrun/resources/results_test.go	Add unit tests for multi-arch result extraction/aggregation.
pkg/reconciler/buildrun/resources/resource_builders.go	Add EmptyDir workspace bindings for multi-arch PipelineRuns.
pkg/reconciler/buildrun/resources/pipelinerun_generator.go	Implement multi-arch PipelineRun generation + scheduling pinning via TaskRunSpecs.
pkg/reconciler/buildrun/resources/multiarch.go	New: multi-arch pipeline task generation (source bundle push/pull, per-platform tasks, assemble-index).
pkg/reconciler/buildrun/resources/multiarch_test.go	New: unit tests for multi-arch pipeline generation contracts.
pkg/reconciler/buildrun/buildrun.go	Reconcile: extract per-platform results from PipelineRun child TaskRuns into BuildRun status.
pkg/image/index.go	New: build an OCI image index from per-platform images with platform descriptors.
pkg/image/index_test.go	Unit tests for index assembly behavior and error cases.
pkg/apis/build/v1beta1/zz_generated.deepcopy.go	DeepCopy updates for new PlatformResults / PlatformBuildResult.
pkg/apis/build/v1beta1/buildrun_types.go	API: define PlatformBuildResult and clarify output digest semantics for multi-arch.
docs/buildrun.md	Document .status.platformResults and multi-arch digest semantics.
docs/build.md	Document spec.output.platforms behavior/requirements and examples.
deploy/crds/shipwright.io_buildruns.yaml	CRD schema updates for new status fields and digest description.
cmd/image-processing/main.go	Add CLI modes for pushing source bundles and assembling/pushing image indexes.
cmd/image-processing/main_test.go	Add tests for new CLI modes and improve short-mode behavior for network-dependent test.

Files not reviewed (1)

• pkg/apis/build/v1beta1/zz_generated.deepcopy.go: Language not supported

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