Update flake8.yml

Author: valentijnscholten

.github/workflows/flake8.yml

@@ -14,6 +14,11 @@ jobs:
   flake8-your-pr:
     runs-on: ubuntu-latest
     steps:
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
       - name: Checkout
         if: github.event_name == 'pull_request' || github.event_name == 'pull_request_target'
         uses: actions/checkout@v2