Update Woodpecker CI config to use existing REGISTRY secrets

.woodpecker.yml

@@ -13,14 +13,14 @@ steps:
     name: build
     image: woodpeckerci/plugin-docker-buildx
     environment:
-      DOCKER_USERNAME:
-        from_secret: docker_username
-      DOCKER_PASSWORD:
-        from_secret: docker_password
+      REGISTRY_USER:
+        from_secret: REGISTRY_USER
+      REGISTRY_PASSWORD:
+        from_secret: REGISTRY_PASSWORD
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
     commands:
-      - echo "$${DOCKER_PASSWORD}" | docker login -u "$${DOCKER_USERNAME}" --password-stdin registry.example.com
+      - echo "$${REGISTRY_PASSWORD}" | docker login -u "$${REGISTRY_USER}" --password-stdin registry.example.com
       - docker build -t registry.example.com/ploughshares:latest -t registry.example.com/ploughshares:${CI_COMMIT_BRANCH} -f docker/ploughshares/Dockerfile .
       - docker push registry.example.com/ploughshares:latest
       - docker push registry.example.com/ploughshares:${CI_COMMIT_BRANCH}