Updated drone config to use the pre-built yarn image

.drone.yml

@@ -13,15 +13,13 @@ steps:
       - git submodule update --recursive --remote
 
   - name: theme
-    image: node:11-alpine
+    image: kolaente/yarn
     pull: true
     group: build-static
     commands:
-      - apk add yarn
       - cd themes/vikunja
-      - yarn global add gulp-cli -g
       - yarn --production=false
-      - gulp
+      - ./node_module/.bin/gulp
 
   - name: build
     image: monachus/hugo