add latest tag to .drone.yml

.drone.yml

@@ -5,7 +5,8 @@ steps:
 - name: write_tags
   image: busybox
   commands:
-    - grep "ENV VERSION" Dockerfile | cut -d '=' -f2 > .tags
+    - echo latest > .tags
+    - grep "ENV VERSION" Dockerfile | cut -d '=' -f2 >> .tags
 
 - name: build_and_deploy_samservices  
   image: plugins/docker