add starlark format linting step

.drone.star

@@ -231,6 +231,7 @@ def main(ctx):
     )
   )
 
+  pipelines.append(checkStarlark)
   pipelineSanityChecks(ctx, pipelines)
   return pipelines
 
@@ -1578,6 +1579,43 @@ def deploy(ctx, config, rebuild):
     },
   }
 
+def checkStarlark():
+    return [{
+        "kind": "pipeline",
+        "type": "docker",
+        "name": "check-starlark",
+        "steps": [
+            {
+                "name": "format-check-starlark",
+                "image": "owncloudci/bazel-buildifier",
+                "pull": "always",
+                "commands": [
+                    "buildifier --mode=check .drone.star",
+                ],
+            },
+            {
+                "name": "show-diff",
+                "image": "owncloudci/bazel-buildifier",
+                "pull": "always",
+                "commands": [
+                    "buildifier --mode=fix .drone.star",
+                    "git diff",
+                ],
+                "when": {
+                    "status": [
+                        "failure",
+                    ],
+                },
+            },
+        ],
+        "depends_on": [],
+        "trigger": {
+            "ref": [
+                "refs/pull/**",
+            ],
+        },
+    }]
+
 def genericCache(name, action, mounts, cache_key):
   rebuild = 'false'
   restore = 'false'