[fix] CI: fix docker push

.github/workflows/integration.yml

@@ -92,7 +92,7 @@ jobs:
       - documentation
     env:
       DOCKERHUB_USERNAME: ${{ secrets.DOCKERHUB_USERNAME }}
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout
         if: env.DOCKERHUB_USERNAME != null