copy files to the host for testing with HTMLProofer

.github/workflows/build-pr.yml

@@ -14,6 +14,11 @@ jobs:
       - uses: actions/checkout@v2
       - name: build image
         run: docker build --target=current -t documentation:latest .
+      - name: copy files to host
+        # https://forums.docker.com/t/is-there-a-way-to-copy-a-file-from-an-image-to-the-local-filesystem-without-running-a-container/2841
+        run: |
+          docker run --name docs documentation:latest /bin/true
+          docker cp docs:/usr/share/nginx/html _site
       - name: check for broken links
         uses: chabad360/htmlproofer@master
         with: