build: copy only Cargo.toml in Dockerfile to leverage caching

Dockerfile

@@ -13,7 +13,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 WORKDIR /app
 
 # Leverage docker layer caching for dependencies
-COPY Cargo.toml Cargo.lock ./
+COPY Cargo.toml ./
 RUN mkdir -p src && echo 'fn main() { println!("cargo:rerun-if-changed=build.rs"); }' > src/main.rs
 RUN cargo build --release || true
 RUN rm -rf src