Ensure stack works when no volumes are specified in the compose file.
diff --git a/docker/client/stack.sls b/docker/client/stack.sls
index dc7a7cc..8bf7ae8 100644
--- a/docker/client/stack.sls
+++ b/docker/client/stack.sls
@@ -56,7 +56,7 @@
           {%- set path = None %}
         {%- endif %}
 
-        {%- if path != None and path not in compose.volume.keys() %}
+        {%- if path != None and path not in compose.get('volume', {}).keys() %}
 docker_{{ app }}_{{ name }}_volume_{{ path }}:
   file.directory:
     - name: {{ path }}