Fix docker tests

There was a quotation mark that was mistakenly removed in #480, but this was not noticed because that PR did not itself trigger a Docker workflow run.  (It runs only on a weekly cron job.)

.github/workflows/docker.yml

@@ -20,4 +20,4 @@ jobs:
         run: docker compose build
       - name: Test notebooks
         shell: bash
-        run: docker compose run notebook "bash" "-c" "pip install pytest nbmake && pytest --nbmake docs
+        run: docker compose run notebook "bash" "-c" "pip install pytest nbmake && pytest --nbmake docs"