Update ci

.drone.yml

@@ -91,6 +91,8 @@ steps:
     volumes:
       - name: lib
         path: /drone/src/docker/backend/lib
+      - name: cache
+        path: /cache
     commands:
       - cd docker/backend
       - make docs
@@ -126,6 +128,9 @@ steps:
 volumes:
   - name: lib
     temp: {}
+  - name: cache
+    host:
+      path: /tmp/cache
   - name: dockersock
     host:
       path: /var/run/docker.sock