Fix secret usage for plugin-docker-buildx

.woodpecker/docker.yaml

@@ -5,10 +5,13 @@ when:
 steps:
   docker:
     image: woodpeckerci/plugin-docker-buildx
-    secrets: [docker_username, docker_password]
     settings:
       pull_image: true
       registry: git.bluemedia.dev
       repo: git.bluemedia.dev/bluemedia/simple-ocpp-cs
+      username:
+        from_secret: docker_username
+      password:
+        from_secret: docker_password
       platforms: linux/amd64
       tag: latest