fix: keep docker mirror on local builder fallback

[pbardea]

Summary

• Generate a BuildKit config with the Docker Hub mirror for fallback docker-container builders.
• Pass that config to docker buildx create when the Blacksmith sticky disk builder setup fails, so fallback builds do not bypass the mirror.
• Add coverage for the fallback config and rebuild the checked-in action bundle.

Test plan

• COREPACK_ENABLE_DOWNLOAD_PROMPT=0 corepack pnpm@10.33.0 test -- src/setup-builder.test.ts
• COREPACK_ENABLE_DOWNLOAD_PROMPT=0 corepack pnpm@10.33.0 typecheck
• COREPACK_ENABLE_DOWNLOAD_PROMPT=0 corepack pnpm@10.33.0 build
• COREPACK_ENABLE_DOWNLOAD_PROMPT=0 corepack pnpm@10.33.0 lint

Made with Cursor

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