Fix dockerhub wrong repo and other misconfigurations

.github/workflows/build_and_push_docker_image.yml

@@ -7,7 +7,7 @@ on:
       OS:
         description: 'OS name (must choose from <centos, ubuntu18, ubuntu20>)'
         required: true
-        default: 'ubuntu18'
+        default: 'ubuntu20'
 
 jobs:
   generate-centos-image:

.github/workflows/demo_test.yml

@@ -341,7 +341,7 @@ jobs:
         submodules: true
 
     - name: Create container
-      run: docker run -itd --name=${{ github.job }} -v $GITHUB_WORKSPACE:/root/occlum zhubojun/occlum:latest-ubuntu20.04-grpc
+      run: docker run -itd --name=${{ github.job }} -v $GITHUB_WORKSPACE:/root/occlum occlumbackup/occlum:latest-ubuntu20.04-grpc
 
     - name: Build dependencies
       run: docker exec ${{ github.job }} bash -c "cd /root/occlum; make submodule"

.github/workflows/main.yml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   Make_test_on_ubuntu:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v1