Dockerfile: update rust version & caching (#35)

Dockerfile

@@ -1,16 +1,22 @@
-FROM rust:1.57-bullseye as builder
+FROM rust:1.62-bullseye as builder
 WORKDIR /usr/src/sdk-rust
 
 # Cache dependencies between test runs,
 # See https://blog.mgattozzi.dev/caching-rust-docker-builds/
 # And https://github.com/rust-lang/cargo/issues/2644
 
 RUN mkdir -p ./plan/src/
-RUN echo "fn main() {}" > ./plan/src/main.rs
+RUN echo "fn main() { println!(\"If you see this message, you may want to clean up the target directory or the Docker build cache.\") }" > ./plan/src/main.rs
 COPY ./plan/Cargo.* ./plan/
 RUN cd ./plan/ && cargo build
 
 COPY . .
+
+# This is in order to make sure `main.rs`s mtime timestamp is updated to avoid the dummy `main`
+# remaining in the release binary.
+# https://github.com/rust-lang/cargo/issues/9598
+RUN touch ./plan/src/main.rs
+
 RUN cd ./plan/ && cargo build --example example
 
 FROM debian:bullseye-slim