fix(docker): avoid double compile in smoke build (#686)

Dockerfile

@@ -12,8 +12,8 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
         pkg-config \
     && rm -rf /var/lib/apt/lists/*
 
-# 1. Copy manifests to cache dependencies
-COPY Cargo.toml Cargo.lock ./
+# 1. Copy manifests and toolchain pin to cache dependencies with the same compiler
+COPY Cargo.toml Cargo.lock rust-toolchain.toml ./
 # Create dummy targets declared in Cargo.toml so manifest parsing succeeds.
 RUN mkdir -p src benches \
     && echo "fn main() {}" > src/main.rs \