add: filter to prevent CI when branch are deleted (#9)

.github/workflows/push.yml

@@ -8,6 +8,8 @@ jobs:
     steps:
     - uses: actions/checkout@master
     - name: Docker build
+      if: github.delete == false
       run: docker build -t peaceiris/actions-hugo .
     - name: Check Hugo version
+      if: github.delete == false
       run: docker run peaceiris/actions-hugo:latest version