added external network for prometheus pushgateway

restic/docker-compose.yml

@@ -14,3 +14,8 @@ services:
     entrypoint: /root/restic/entry.sh 
     # For debugging, docker-compose run restic
     #entrypoint: /bin/sh
+    networks:
+      - prometheus_pushgateway
+networks:
+  prometheus_pushgateway:
+    external: true