add configure-docker step

.github/workflows/release-slackbot-proxy.yml

@@ -40,6 +40,10 @@ jobs:
         service_account_key: ${{ secrets.GCP_SA_KEY_SLACKBOT_PROXY }}
         export_default_credentials: true
 
+    - name: Configure docker for gcloud
+      run: |
+        gcloud auth configure-docker
+
     - name: Build and push
       uses: docker/build-push-action@v2
       with: