Fix Docker Hub credentials

Author: stefanprodan

.github/workflows/release.yml

@@ -24,8 +24,8 @@ jobs:
       - name: Push image
         uses: docker/build-push-action@v1
         with:
-          username: ${{ secrets.DOCKER_USERNAME }}
-          password: ${{ secrets.DOCKER_PASSWORD }}
+          username: ${{ secrets.DOCKER_FLUXCD_USER }}
+          password: ${{ secrets.DOCKER_FLUXCD_PASSWORD }}
           repository: fluxcd/notification-controller
           tag_with_ref: true
       - name: Create release