The OPENSHELL_CARGO_VERSION build arg contains a git commit hash that
changes on every build (e.g. 0.0.7-dev.11+g085b131ae). Declaring this
ARG near the top of each Dockerfile invalidated every layer below it --
including expensive dependency installs, toolchain setup, and the Rust
dependency pre-build step -- on every single commit.
Move the ARG declaration to just before the RUN that actually uses it so
upstream layers stay cached. This recovers ~5-6 minutes per build on the
gateway image (from ~9m back toward ~2-3m) and similarly improves cluster,
CLI, and Python wheel builds.
Also removes unused OPENSHELL_IMAGE_TAG ARG from cli-macos, python-wheels,
and python-wheels-macos Dockerfiles.
## Summary
- add a Docker-based macOS arm64 wheel build path so darwin wheels can be produced from Linux CI hosts
- wire macOS wheel builds into release publish workflows (`publish:main` and `publish:tag`) alongside existing Linux wheel builds
- mirror `crazymax/osxcross:latest` into `$CI_REGISTRY_IMAGE/third_party/osxcross:latest` and use that mirrored image in CI publish jobs
## Scope Notes
- this MR focuses on shipping reliable macOS wheel build support first
- temporary MR-only smoke-job wiring was removed after validation so the branch reflects final intended CI behavior
- broader cache optimization work is tracked separately in issue #43
## Test Plan
- run `mise run pre-commit`
- validate `.gitlab-ci.yml` with `uv run --with pyyaml python -c 'import yaml, pathlib; yaml.safe_load(pathlib.Path(".gitlab-ci.yml").read_text())'`
Piotr Mlocek