Merge pull request #28 from xcad2k/xcad2k/issue27

portainer-data volume declaration is missing in compose file

docker-compose/portainer/docker-compose.yml

@@ -1,5 +1,9 @@
 version: '3'
 
+volumes:
+  portainer-data:
+    driver: local
+    
 services:
   app:
     container_name: portainer