perf(backend): Cache build of dependencies

When building the Docker image, first compile the project with a fake
main file, namely `dummy.rs`. Once dependencies are built, compile the
actual project by switching back to `src/main.rs`.

Signed-off-by: Lucien Cartier-Tilet <lucien@phundrak.com>

Dockerfile

@@ -2,12 +2,20 @@ FROM rust:alpine
 
 WORKDIR /app
 
-COPY Cargo.toml /app/Cargo.toml
-COPY Cargo.lock  /app/Cargo.lock
-COPY src /app/src
 RUN apk update && apk add pkgconfig openssl-dev musl-dev
 
+COPY Cargo.toml .
+COPY dummy.rs .
+
+# Cache building
+RUN sed -i 's|src/main.rs|dummy.rs|' Cargo.toml
 RUN cargo build --release
+RUN cargo build
+RUN sed -i 's|dummy.rs|src/main.rs|' Cargo.toml
+
+# Building normally
+COPY src src
+RUN cargo build
 RUN cargo install --path .
 
 ENTRYPOINT [ "phuncache" ]