Add the extra.symfony.docker flag

docker/app/docker-entrypoint.sh

@@ -16,6 +16,10 @@ if [ "$1" = 'php-fpm' ] || [ "$1" = 'bin/console' ]; then
     # The first time volumes are mounted, the project needs to be recreated
     if [ ! -f composer.json ]; then
         composer create-project "symfony/skeleton $SYMFONY_VERSION" tmp --stability=$STABILITY --prefer-dist --no-progress --no-interaction
+        jq '.extra.symfony.docker=true' tmp/composer.json > tmp/composer.tmp.json
+        rm tmp/composer.json
+        mv tmp/composer.tmp.json tmp/composer.json
+
         cp -Rp tmp/. .
         rm -Rf tmp/
     elif [ "$APP_ENV" != 'prod' ]; then