[Request]: Support building for exotic architectures (ppc64le, s390x, riscv64) via QEMU emulation in the build VM

### Feature or enhancement request details

`container build --platform linux/` works for `linux/arm64` (native) and `linux/amd64` (via Rosetta — as reflected in `build.rosetta` system property). However, building images that require `RUN` instructions for other architectures fails with `Exec format error`:

```console
% container build --platform linux/ppc64le -t myimage:ppc64le .
#4 [linux/ppc64le 1/2] RUN echo "arch: $(uname -m)" > /arch.txt
#4 ERROR: failed to unmount ...: operation not permitted: failed to mount ...: mount source: "/var/lib/buildkit/runc-native/snapshots/...", fstype: bind, flags: 20480, err: operation not permitted
Error: failed to solve: Build failed with exit code 1
```

Dockerfiles that contain only `FROM` + `COPY` + `CMD` (no `RUN`) do work, because those just repackage existing layers from the multi-arch base image. The failure occurs when any `RUN` instruction must execute a foreign-arch binary.

**Verified failing architectures**: `linux/ppc64le`, `linux/s390x`, `linux/riscv64` (see also #809, which covers `riscv64` for `container run`).

### macOS container tooling landscape

This gap affects the broader macOS container ecosystem, not just `container`. For context:

| Tool | `linux/arm64` | `linux/amd64` | `linux/ppc64le` / `linux/s390x` |
|------|:---:|:---:|:---:|
| `container build` (this project) | ✅ native | ✅ Rosetta | ❌ Exec format error |
| `podman build` via Podman machine | ✅ native | ✅ QEMU | ✅ QEMU (Fedora CoreOS VM has `qemu-user-static`) |
| Docker Desktop | ✅ native | ✅ Rosetta | ✅ QEMU |

Podman machine on macOS runs a Fedora CoreOS QEMU VM which ships `qemu-user-static` binfmt handlers — this is why `podman build --platform linux/ppc64le` with `RUN` steps works on macOS today, at the cost of a persistent shared QEMU VM. `container` and Docker Desktop (via Rosetta for amd64) both take a lighter-weight approach that doesn't currently extend to exotic architectures.

### Why this matters

IBM Power (ppc64le) and IBM Z (s390x) are major enterprise Linux targets. Many open-source projects — including OpenShift, Kubernetes operators, and Red Hat middleware — require multi-arch images that include these platforms. Developers on Apple Silicon currently have no path to build images with `RUN` steps for these architectures using `container` without a remote Linux builder.

### Proposed solution

Add QEMU user-mode static emulation support inside the BuildKit builder VM. This is how Docker Desktop and standard Linux CI (via `tonistiigi/binfmt`) handles cross-arch `RUN` steps:

1. Register `qemu-*-static` binfmt handlers inside the build VM at builder startup
2. Make this opt-in via a new system property (e.g. `build.qemu Bool false Enable QEMU user-mode emulation for exotic-arch RUN steps`) similar to how `build.rosetta` toggles Rosetta for amd64

Precedent: `build.rosetta` already shows the pattern of toggling arch-specific emulation strategies.

### Current workarounds

- **Remote builder**: `docker buildx create --driver docker-container --platform linux/ppc64le ssh://user@linux-box` — requires a separate Linux machine with QEMU binfmt configured
- **Podman machine**: `podman build --platform linux/ppc64le` works today via the Fedora CoreOS QEMU VM — at the cost of a persistent shared VM
- **GitHub Actions**: `docker/setup-qemu-action` on Ubuntu runners — zero-infrastructure path for CI
- **IBM Cloud**: Real Power/Z VSIs for production builds

### Environment

- macOS 26.0, Apple Silicon (M-series)
- `container` CLI (current)

Related: #809 (riscv64 `Exec format error` on `run`, closed), #913 (multi-platform semantics umbrella)

> [!Note]
> Responses generated with Claude

Start at the `container build --platform` entry point and trace the existing `build.rosetta` system property through BuildKit builder VM startup. Determine where opt-in QEMU user-mode handlers would be registered, then verify that `RUN` steps work for ppc64le, s390x, and riscv64 while existing native and Rosetta builds remain unchanged.

Written by the indexing model from the issue text.