docker: save a few minutes of build time

Dockerfile: install rust before copying the source tree to save a few minutes of build time.

release/docker/Dockerfile

@@ -18,12 +18,12 @@ RUN apk update && \
 
 WORKDIR /duniter
 
-# copy source tree
-COPY ./ ./
-
 # install latest stable rust
 RUN curl https://sh.rustup.rs -sSf | sh -s -- -y
 
+# copy source tree
+COPY ./ ./
+
 # build duniter
 RUN PATH=${HOME}/.cargo/bin:${PATH} \
 	RUSTFLAGS="-C target-feature=-crt-static -L/usr/lib" \