ci: Move libvalhalla into a separate cache layer

Dockerfile

@@ -25,16 +25,19 @@ WORKDIR /usr/src/app
 COPY Cargo.toml Cargo.lock ./
 RUN mkdir src && echo 'fn main() { println!("Dummy image called!"); }' > src/main.rs
 # And for every other target in the workspace
-RUN mkdir -p libvalhalla/src && echo '' > libvalhalla/src/lib.rs
 COPY libvalhalla/Cargo.toml ./libvalhalla/
+RUN mkdir -p libvalhalla/src && touch libvalhalla/src/lib.rs
+RUN cargo build --release
+
+# libvalhalla compilation takes a lot, worth to move it into a separate cache layer
+COPY libvalhalla ./libvalhalla
+RUN touch -a -m ./libvalhalla/src/lib.rs
 RUN cargo build --release
 
 # Now build the real target
 COPY src ./src
-COPY libvalhalla ./libvalhalla
 # Update modified attribute as otherwise cargo won't rebuild it
 RUN touch -a -m ./src/main.rs
-RUN touch -a -m ./libvalhalla/src/lib.rs
 RUN cargo build --release
 
 FROM debian:bookworm-slim AS runner