Update Rust build system: use rustup with musl target for static builds

build/Dockerfile.cached

@@ -33,10 +33,15 @@ RUN apk add --no-cache \
     grep \
     findutils \
     jq \
-    # Go and Rust for source compilation
+    # Go for source compilation
     go \
-    rust \
-    cargo
+    # Rustup for proper Rust musl builds
+    rustup
+
+# Setup Rust toolchain for musl builds
+RUN rustup-init -y --default-toolchain stable && \
+    . ~/.cargo/env && \
+    rustup target add x86_64-unknown-linux-musl
 
 # Create standard directories
 RUN mkdir -p /build/initramfs /build/kernel /build/output /build/github /build/configs/zinit /mnt/zinit /build/source /build/cache