nihil build fails on ARM64 hosts: docker build is missing --platform

[0xbbuddha]

Problem

nihil build fails on ARM64 hosts (Apple Silicon, ARM Linux).

_cmd_build shells out to docker build with no --platform flag (nihil/cli/controller.py:1217), and all our Dockerfiles are based on FROM archlinux:latest, which upstream publishes for linux/amd64 only. On an ARM64 host the build aborts while resolving the base image, with the same error #5 fixed for pulls:

no matching manifest for linux/arm64/v8 in the manifest list entries

This is the third code path needing an explicit platform hint. #5 covered the other two (image pull and container creation) but deliberately scoped build out.

Suggested fix

Reuse get_image_platform(), added in #5, in _cmd_build when assembling the docker build argv:

image_platform = get_image_platform()
if image_platform:
    cmd += ["--platform", image_platform]

Note that the same import-cycle constraint applies as in manager.py: import get_image_platform inside the function, not at module scope, otherwise nihil.utils -> doctor -> nihil.manager becomes circular.

Acceptance

• nihil build <variant> completes on an ARM64 host with emulation available (Rosetta or QEMU).
• Behaviour on amd64 hosts is unchanged, no --platform passed.
• Unit test asserting the flag is present on ARM and absent on amd64, mirroring the existing tests in tests/test_nihilManager.py.

Notes

Worth checking whether nihil build --tag and the blueteam / test variants need anything beyond this.

@whiteov3rflow this follows on from your #5 and you already have the ARM setup to verify it, so it's yours if you want it. No obligation either way.

[whiteov3rflow]

Thanks for the follow-up. I’ll take this and validate the build path on Apple Silicon with OrbStack, including ARM64/amd64 argument tests and the tagged build variants.