Add LUA linting

.drone.yml

@@ -13,9 +13,8 @@ steps:
   - cd sources && git checkout $DRONE_COMMIT
 
 - name: lint
-  image: alpine
+  image: martijnrondeel/luarocks
   commands:
-    - apk update && apk add --no-cache luarocks
     - luarocks install luacheck
     - luacheck sources