Add APP_SECRET to Dockerfile

docker/php/Dockerfile

@@ -90,9 +90,9 @@ CMD ["php-fpm"]
 # -----------------------------------------------------------------------------
 FROM base AS prod
 ARG APP_ENV=prod
+ARG BUILD_APP_SECRET=build-secret
 ENV APP_ENV=${APP_ENV}
 ENV APP_DEBUG=0 \
-    APP_SECRET=build-secret \
     DATABASE_DRIVER=sqlite \
     DATABASE_SQLITE_PATH=/var/www/html/var/data/database.sqlite \
     RUN_MIGRATIONS_ON_START=1
@@ -112,6 +112,7 @@ COPY . /var/www/html
 # Finalize install & warm cache
 RUN if [ -f bin/console ]; then \
       set -ex; \
+      export APP_SECRET="${BUILD_APP_SECRET}"; \
       composer dump-autoload --no-dev --optimize; \
       php bin/console cache:clear --no-warmup; \
       php bin/console cache:warmup; \