add overlay network to pushkin

issue: https://gerrit.mcp.mirantis.net/#/c/9662/2/docker/swarm/stack/pushkin.yml
Change-Id: Ib1de4963df79f87d2a55d132479d2fda763119fe
diff --git a/docker/swarm/stack/pushkin.yml b/docker/swarm/stack/pushkin.yml
index 13198b7..71b5f5f 100644
--- a/docker/swarm/stack/pushkin.yml
+++ b/docker/swarm/stack/pushkin.yml
@@ -38,3 +38,7 @@
                 - ${_param:haproxy_pushkin_exposed_port}:${_param:haproxy_pushkin_bind_port}
               volumes:
                 - /srv/volumes/pushkin/api:/var/log/pushkin
+          network:
+            default:
+              external:
+                name: oss_backend