feat(build): forward --builder to container build (refs #10)

[us]

Summary

Wires the previously-stubbed --builder flag through to container build, and
documents the remote-builder workaround for exotic architectures. Two of the four
acceptance criteria on #10 that are achievable without waiting on upstream.

⚠️ This does not close #10. The core problem — the local arm64 BuildKit VM
has no QEMU binfmt handlers for ppc64le/s390x/riscv64 RUN steps — is
upstream-blocked on apple/container#1496.
#10 stays open as the tracking issue.

What changed

• Wire --builder end-to-end. Build.run() now forwards builder: to
  ImageManager.build(), which gained a builder: String? parameter that maps
  to container build --builder. Previously the flag was accepted, shown in
  --help, and silently discarded.
• Extract makeBuildArguments() — a pure, side-effect-free static helper
  that builds the container build argument vector. Makes argument construction
  unit-testable without spawning a process.
• README: added a third row to the exotic-arch workarounds table covering the
  remote-builder path (mocker build --builder <name> --platform linux/ppc64le,
  backed by a remote BuildKit node such as docker buildx create --driver remote ssh://host).

Why this is the right scope

Per the investigation in .context/issue-10-report.md: Options A (Podman + remote
BuildKit auto-routing) and B/C remain deferred — not locally testable and/or
multi-PR. Option D (manual remote builder) is the cleanest architecture and the
--builder wiring is its mechanical first step, usable today.

Testing

• New ImageManagerBuildArgsTests (6 tests): verifies --builder is forwarded
  with its value, omitted when nil/empty, composes with --platform linux/ppc64le,
  and that the full flag matrix is well-formed.
• swift build — clean.
• swift test — full suite green (71 tests, 9 suites).
• mocker build --help confirms --builder is surfaced.

Refs #10