Normalize Compose service additional contexts in bake

[PedroGuerraPT]

Summary

• Translate Compose additional_contexts values like service:api.docs into generated bake target links like target:api_docs.
• Preserve non-service additional contexts unchanged.
• Add regression coverage for linked Compose build targets.

Test plan

• GOCACHE=/private/tmp/go-cache go test ./pkg/buildx/bake

Support thread: T-9318

---

Note

Low Risk
Scoped to compose bake target generation and additional context mapping; behavior change only affects service: additional context values.

Overview
Compose-to-bake conversion now rewrites additional_contexts entries that reference another service (e.g. service:api.docs) into bake target: links that match sanitized target names (e.g. target:api_docs). Path and other non-service values are left unchanged.

Service-to-target naming is built once via composeBuildTargetNames and reused when emitting targets, so linked contexts stay consistent with how compose services become bake targets (including dot-to-underscore sanitization). A regression test covers a service-linked context plus a local path context.

^{Reviewed by Cursor Bugbot for commit 138f26b. Bugbot is set up for automated code reviews on this repo. Configure here.}

[PedroGuerraPT]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: 138f26b37c

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[PedroGuerraPT]

@BugBot review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit 138f26b. Configure here.}