chore(docker): ensure resilient multi-arch detection and explicit ziglang dependency

Dockerfile

@@ -18,11 +18,11 @@ ARG TARGETARCH
 # zig (via the cargo-zigbuild wheel) is the cross C toolchain for musl targets.
 RUN apt-get update && apt-get install -y --no-install-recommends python3-pip \
  && rm -rf /var/lib/apt/lists/* \
- && pip install --no-cache-dir --break-system-packages cargo-zigbuild
-RUN case "$TARGETARCH" in \
-      amd64) echo x86_64-unknown-linux-musl ;; \
-      arm64) echo aarch64-unknown-linux-musl ;; \
-      *) echo "unsupported arch $TARGETARCH" >&2; exit 1 ;; \
+ && pip install --no-cache-dir --break-system-packages cargo-zigbuild ziglang
+RUN case "${TARGETARCH:-$(uname -m)}" in \
+      amd64|x86_64) echo x86_64-unknown-linux-musl ;; \
+      arm64|aarch64) echo aarch64-unknown-linux-musl ;; \
+      *) echo "unsupported arch ${TARGETARCH:-$(uname -m)}" >&2; exit 1 ;; \
     esac > /rust-target \
  && rustup target add "$(cat /rust-target)"
 WORKDIR /src