update preffix, in order to avoid pkg conflicts on pool
diff --git a/aptly/server/mirror/ubuntu/xenial/docker.yml b/aptly/server/mirror/ubuntu/xenial/docker.yml
index cfaaa32..d420dfe 100644
--- a/aptly/server/mirror/ubuntu/xenial/docker.yml
+++ b/aptly/server/mirror/ubuntu/xenial/docker.yml
@@ -12,4 +12,4 @@
           publisher:
             component: docker
             distributions:
-              - nightly/xenial
+              - xenial/nightly