fix(bootstrap): set docker build platform args

[shiju-nv]

Summary

Make Dockerfile-based sandbox image builds pass explicit Docker BuildKit platform metadata. It uses TARGETPLATFORM for the requested output image when supplied, falls back to the Docker daemon platform, and seeds the implicit TARGET*/BUILD* build args so Dockerfile behavior does not depend on daemon defaults.

Related Issue

N/A

Changes

• Query the local Docker daemon platform and normalize Docker-reported architecture names.
• Set the Docker build request platform from the effective target platform.
• Seed BuildKit implicit platform args without overwriting caller-provided build args.
• Avoid injecting empty TARGETVARIANT or BUILDVARIANT values so Dockerfile defaults still work for no-variant platforms.
• Add unit coverage for native builds, cross-platform builds, architecture normalization, and malformed platform handling.

Testing

• mise run pre-commit passes
• Unit tests added/updated
  • cargo fmt --check
  • env CARGO_TARGET_DIR=/Users/shiju/.cargo/shared-target cargo test -p openshell-bootstrap
• E2E tests added/updated (if applicable)
  • Not applicable for this narrowly scoped bootstrap build-argument change; covered by focused unit tests.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)
  • Not applicable; no architecture boundary or user-facing configuration changed.

[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[shiju-nv]

I have read the DCO document and I hereby sign the DCO.

[drew]

recheck

[drew]

/ok to test bfac8a5