Use default keyring in aptly

Change-Id: Ie34fd403fd2a74aace2ff9b567b31bb8ed16ea95
diff --git a/aptly/server/docker.yml b/aptly/server/docker.yml
index ee8f4a2..3fa0a8e1 100644
--- a/aptly/server/docker.yml
+++ b/aptly/server/docker.yml
@@ -20,7 +20,6 @@
       root_dir: /srv/volumes/aptly
       no_config: true
       gpg:
-        keyring: .gnupg/trustdb.gpg
         keypair_id: ${_param:aptly_gpg_keypair_id}
         public_key: ${_param:aptly_gpg_public_key}
         private_key: ${_param:aptly_gpg_private_key}