cache dependencies in docker build

Dockerfile

@@ -1,7 +1,15 @@
 FROM ekidd/rust-musl-builder AS build
 
-ADD . ./
-RUN sudo chown -R rust:rust .
+COPY Cargo.toml Cargo.lock ./
+
+# Build with a dummy main to pre-build dependencies
+RUN mkdir src && \
+ echo "fn main(){}" > src/main.rs && \
+ cargo build --release && \
+ rm -r src
+
+COPY src/* ./src/
+RUN sudo chown -R rust:rust . && touch src/main.rs
 
 RUN cargo build --release