Update actions/checkout action to v2

Author: renovate-bot

.github/workflows/push.yml

@@ -27,7 +27,7 @@ jobs:
     steps:
     - id: git-checkout
       name: Checkout
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
     - id: docker-build
       name: Build the image from '${{ matrix.docker_from }}' with '${{ matrix.build_cmd }}'
       run: ${{ matrix.build_cmd }}