fix(docker): Start with lib project for caching

Dockerfile

@@ -2,6 +2,7 @@ FROM ekidd/rust-musl-builder:stable as builder
 
 # create new cargo project
 RUN USER=rust cargo init --lib
+RUN echo 'fn main() { println!("Hello, world!"); }' >> src/main.rs
 # copy build config
 COPY --chown=rust ./Cargo.lock ./Cargo.lock
 COPY ./Cargo.toml ./Cargo.toml