v0.10.1: drop Windows from release matrix; simplify Dockerfile

[guyte149]

Summary

Two release-pipeline fixes shaken out by the first v0.10.0 tag run.

Drop Windows from the release matrix

x86_64-pc-windows-msvc failed to build:

error[E0432]: unresolved import `tokio::net::UnixListener`
error[E0599]: no method named `set_mode` found for struct `Permissions`
   --> src/server/admin.rs:85:11

src/server/admin.rs uses tokio::net::UnixListener and POSIX Permissions::set_mode unconditionally — both Unix-only. This is a real product limitation, not a workflow tweak: properly supporting Windows means #[cfg(unix)]-gating the entire admin API module (or backing it with a Windows named pipe). Out of scope here. README updated to call out the limitation explicitly.

Simplify the Dockerfile

The multi-arch GHCR job failed with:

buildx failed: failed to compute cache key: "/usr/local/bin/rusnel": not found

Root cause: the previous Dockerfile used BuildKit cache mounts on target/ plus a cross-compile path for arm64. The cache mount was unmounted before the multi-stage COPY --from=builder could resolve the binary, so the cache key calculation found nothing.

Replaced with a plain cargo build --release under buildx + QEMU emulation. Each arch goes through the same build path; the cost is slower arm64 builds (a few minutes under QEMU) and the benefit is a Dockerfile that actually works and that any contributor can reproduce locally with one command. Final image is `gcr.io/distroless/cc-debian12:nonroot` (~30 MB, runs unprivileged).

Notes

• v0.10.0 release stays as it is — its 5 successful binaries (Linux x86_64 gnu/musl, aarch64-musl, both macOS arches) are already attached to the existing GitHub Release. After this merges I'll tag `v0.10.1` to re-run the workflow with the fixes.
• Version bumped 0.10.0 → 0.10.1 (patch — release-infra fix only).

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