Add Dockerfile for OpenWRT SDK with Rust support

Dockerfile

@@ -0,0 +1,9 @@
+ARG SAFE_TARGET
+ARG VERSION
+
+FROM ghcr.io/openwrt/sdk:${SAFE_TARGET}-${VERSION}
+
+RUN ./scripts/feeds update packages && \
+    make package/feeds/packages/rust/host-compile -j$(nproc) && \
+    rm -rf build_dir/host/rustc-*/rustc-*/build \
+           dl/rustc-*.tar.*