ci(docker-build): fix `when_path`

.woodpecker/docker-build.yml

@@ -1,8 +1,7 @@
 variables:
   - &buildx woodpeckerci/plugin-docker-buildx:2.0.0
   - &platforms linux/amd64
-  - &when_path
-    - ".woodpecker/docker-build.yml"
+  - &when_path .woodpecker/docker-build.yml
 
 matrix:
   DOCKERFILE:
@@ -20,7 +19,7 @@ pipeline:
     when:
       event: pull_request
       path:
-        << *when_path
+        - *when_path
         - ${DOCKERFILE}
 
   publish:
@@ -38,7 +37,7 @@ pipeline:
     when:
       event: push
       path:
-        << *when_path
+        - *when_path
         - ${DOCKERFILE}
 
 branches: master