chore: update docker plugin.

Author: appleboy

.drone.yml

@@ -238,7 +238,7 @@ pipeline:
       branch: [ master ]
 
   docker:
-    image: plugins/docker:17.05
+    image: plugins/docker:17.12
     pull: true
     secrets: [ docker_username, docker_password ]
     repo: gitea/gitea
@@ -248,7 +248,7 @@ pipeline:
       branch: [ release/* ]
 
   docker:
-    image: plugins/docker:17.05
+    image: plugins/docker:17.12
     pull: true
     repo: gitea/gitea
     default_tags: true