optimize docker build

Dockerfile

@@ -1,9 +1,19 @@
 FROM ekidd/rust-musl-builder AS build
 
-ADD . ./
-RUN sudo chown -R rust:rust .
+COPY Cargo.toml Cargo.lock ./
 
-RUN cargo build --release
+# 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/
+COPY sqlx-data.json ./
+
+RUN sudo chown -R rust:rust . && \
+ touch src/main.rs && \
+ cargo build --release
 
 FROM alpine:latest