Update docker-publish.yml read user and pw from env

Author: velo

.github/workflows/docker-publish.yml

@@ -33,8 +33,8 @@ jobs:
       - name: Log in to Docker Hub
         uses: docker/login-action@v2
         with:
-          username: ${{ secrets.DOCKER_USERNAME }}
-          password: ${{ secrets.DOCKER_PASSWORD }}
+          username: ${{ env.DOCKER_USERNAME }}
+          password: ${{ env.DOCKER_PASSWORD }}
 
       - name: Build and push Docker image
         working-directory: flink-dockerfile-example