mount the visual search image into the container

visual_search.jpg is gitignored and excluded by .dockerignore, so the
container had no file at the path rewards_tasks.py uploads and the visual
search task was the one task that could not run in it. Bind mount it from
the project root, where src/random_image_for_visual_search.py writes it.

Also add the blank line the Logging heading needs to render.

docker-compose.yml

@@ -27,3 +27,8 @@ services:
     volumes:
       # Keeps the sign-in across container rebuilds.
       - ./data-dir:/app/data-dir
+      # The visual search task uploads this file, which is gitignored and so is
+      # never in the image. Create it first with
+      # `python src/random_image_for_visual_search.py`; a path that does not
+      # exist yet is mounted as an empty directory rather than a file.
+      - ./visual_search.jpg:/app/visual_search.jpg:ro