move src code copy to second stage

this enables better caching in case the dependencies
stay the same and the only actual code changed

Dockerfile

@@ -2,7 +2,7 @@ FROM docker.io/composer AS composer
 
 WORKDIR /app
 
-COPY composer.json main.php functions.php ./
+COPY composer.json ./
 
 RUN composer install
 
@@ -11,6 +11,7 @@ RUN composer install
 FROM docker.io/php:alpine
 
 COPY --from=composer /app /app
+COPY main.php functions.php ./
 
 WORKDIR /app