Don't call grunt directly, thus decoupling it from the build process.

.drone.yml

@@ -8,7 +8,5 @@ steps:
       - npm install
   - name: build
     image: node:12
-    environment:
-      NPMPATH: "node_modules/.bin"
     commands:
-      - $NPMPATH/grunt
+      - npm run build

package.json

@@ -6,6 +6,9 @@
     "jquery-plugin",
     "qlocktwo"
   ],
+    "scripts": {
+        "build": "grunt"
+    },
   "homepage": "http://bärneruhr.ch/",
   "author": {
     "name": "Manuel Friedli",