Put actions/checkout in reusable workflow

Author: hebasto

.github/workflows/run-in-docker.yml

@@ -15,6 +15,9 @@ jobs:
   reusable_workflow_job:
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      
       - name: Set environment variables
         run: |
           read -r -a vars <<< "${{ matrix.configuration.env_vars }}"