Fix cargo-chef prepare

In prepare stage, cargo-chef reads Cargo.lock and transforms it accordingly.
If Cargo.lock is not present, cargo-chef will generate a new one first, which
might vary a lot and invalidate docker build caches.

Dockerfile

@@ -5,6 +5,7 @@ WORKDIR /usr/src
 ARG CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse
 
 FROM chef as planner
+COPY Cargo.lock Cargo.lock
 COPY Cargo.toml Cargo.toml
 COPY rust-toolchain.toml rust-toolchain.toml
 COPY proto proto