Fix deprecated secrets syntax in Woodpecker CI config

.woodpecker.yml

@@ -1,6 +1,11 @@
 steps:
   - name: build
     image: woodpeckerci/plugin-docker-buildx
+    environment:
+      DOCKER_USERNAME:
+        from_secret: docker_username
+      DOCKER_PASSWORD:
+        from_secret: docker_password
     settings:
       repo: registry.example.com/ploughshares
       registry: registry.example.com
@@ -12,7 +17,6 @@ steps:
       platforms:
         - linux/amd64
         - linux/arm64
-    secrets: [ docker_username, docker_password ]
     when:
       event: [push, tag]
       branch: [main, master]