fix GH actions docker build

if this works, how did the build ever work

.github/workflows/docker.yml

@@ -23,6 +23,8 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
+      - uses: actions/checkout@v2
+
       - name: Build and push Docker image
         uses: docker/build-push-action@v2
         with: