Looks like somebody forgot to add brackets.

.woodpecker/dockerize.yml

@@ -1,7 +1,7 @@
 pipeline:
   dockerize:
     when:
-      branch: main, master
+      branch: [ main, master ]
       event: push
     image: woodpeckerci/plugin-docker-buildx
     settings:
@@ -14,4 +14,4 @@ pipeline:
         from_secret: user
       password:
         from_secret: passwd
-    secrets: [user, passwd]
+    secrets: [ user, passwd ]