feat: add DB_DEFAULT_USER enviroment entry to backend service

create an entry in the enviroment of the backend service
and set it as ${DB_DEFAULT_USE:-kernelci}

docker-compose.yml

@@ -42,6 +42,7 @@ services:
             - 'ALLOWED_HOSTS=["backend", "localhost"]'
             - DB_DEFAULT_PASSWORD_FILE=/run/secrets/postgres_password_secret
             - DB_DEFAULT_HOST=cloudsql-proxy
+            - DB_DEFAULT_USER=${DB_DEFAULT_USER:-kernelci}
 
     dashboard:
         build: ./dashboard