Test that persistent-volume is writable under Docker Compose (#550)

* Test that `persistent-volume` is writable under Docker Compose

The Docker test is expected to fail before #548 is merged and to succeed after it is merged.

* Run docker tests when `workflows/docker.yml` changes

.github/workflows/docker.yml

@@ -3,10 +3,10 @@ name: Docker notebook tests
 on:
   push:
     branches: [ main ]
-    paths: ['Dockerfile', '.dockerignore', 'compose.yaml']
+    paths: ['Dockerfile', '.dockerignore', 'compose.yaml', '.github/workflows/docker.yml']
   pull_request:
     branches: [ main ]
-    paths: ['Dockerfile', '.dockerignore', 'compose.yaml']
+    paths: ['Dockerfile', '.dockerignore', 'compose.yaml', '.github/workflows/docker.yml']
   schedule:
     - cron: '0 20 * * 3'
 
@@ -21,3 +21,6 @@ jobs:
       - name: Test notebooks
         shell: bash
         run: docker compose run notebook "bash" "-c" "pip install pytest nbmake && pytest --nbmake docs"
+      - name: Test that persistent-volume is writable
+        shell: bash
+        run: docker compose run notebook "bash" "-c" "touch persistent-volume/empty-notebook.ipynb"