Add build-essential for Rust/cargo linker

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: centdix

sandbox-image/Dockerfile.sandbox

@@ -2,7 +2,7 @@ FROM debian:bookworm-slim
 
 # ── System packages ───────────────────────────────────────────────────────────
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    curl ca-certificates git unzip openssh-client \
+    curl ca-certificates git unzip openssh-client build-essential \
     && rm -rf /var/lib/apt/lists/*
 
 # ── Node.js 22 ────────────────────────────────────────────────────────────────