chore: support SYMFONY_REQUIRE in local docker env

Author: adrienbrault

Dockerfile

@@ -7,4 +7,8 @@ RUN sudo apt-get update \
     && sudo apt-get install -y make \
     && sudo rm -rf /var/lib/apt/lists/*
 
+# Install symfony/flex for SYMFONY_REQUIRE
+RUN composer global config --no-plugins allow-plugins.symfony/flex true \
+    && composer global require --no-progress --no-scripts --no-plugins symfony/flex
+
 ENV PATH="${PATH}:/usr/src/app/vendor/bin"