Moved the full install to a second build step

Dockerfile

@@ -1,7 +1,12 @@
-FROM composer
+FROM composer AS base
 RUN apk add postgresql-dev openldap-dev \
  && docker-php-ext-install pdo mbstring pdo_pgsql ldap
 
 WORKDIR /app
 COPY src/composer.json src/composer.lock /app/
 RUN composer install --no-autoloader
+
+FROM composer AS install
+COPY ./src /app
+COPY --from=base /app/vendor /app/vendor
+RUN composer install