build(elixir/phoenix): refactor dockerfile for faster rebuild (#7610)

frameworks/Elixir/phoenix/phoenix.dockerfile

@@ -8,14 +8,17 @@ ARG MIX_ENV="prod"
 RUN mix local.hex --force && \
     mix local.rebar --force
 
+COPY mix.exs mix.lock ./
+RUN mix deps.get --force --only prod
+
 COPY config ./config
-COPY lib ./lib
-COPY rel ./rel
+
+RUN mix deps.compile
+
 COPY priv ./priv
-COPY mix.exs .
-COPY mix.lock .
+COPY lib ./lib
 
-RUN mix deps.get --force --only prod
+COPY rel ./rel
 RUN mix release --force --path /export
 
 ####################