fix: set default timeout for GUNICORN workers

Author: tudoramariei

docker/s6-rc.d/backend/run

@@ -33,5 +33,5 @@ else
     --log-level "${LOG_LEVEL}" \
     --worker-class gevent \
     --workers "${WORKERS}" \
-    --timeout "${GUNICORN_WORKERS_TIMEOUT:60}"
+    --timeout "${GUNICORN_WORKERS_TIMEOUT:-60}"
 fi