Try setting a user in the php-fpm pool

.gitlab-ci.yml

@@ -19,6 +19,7 @@ test:
     SITE_URL: "http://noblogs:8080"
     MYSQL_DATABASE: noblogstest
     MYSQL_ROOT_PASSWORD: changeme
+    PHP_FPM_USER: www-data
   script:
     - apt -q update
     - env DEBIAN_FRONTEND=noninteractive apt -y --no-install-recommends install mariadb-client curl

docker/conf/php/7.3/fpm/pool.d/www.conf

 [www]
+user = ${PHP_FPM_USER}
 listen = /run/php/php7.3-fpm.sock
 
 pm = dynamic