docs(docker): Clarify where the configuration is

The docs move from installation with Docker straight to modifying the configuration
without detailing where those files are.

docs/source/guides/install/docker.rst

@@ -151,4 +151,12 @@ If you prefer to only run the container once, you can use the CLI command instea
     -v <OPTIONAL/path/to/movies>:/movies \
     -v <OPTIONAL/path/to/music_videos>:/music_videos \
     -v <OPTIONAL/path/to/music>:/music \
-    ghcr.io/jmbannon/ytdl-sub-gui:latest
+    ghcr.io/jmbannon/ytdl-sub-gui:latest
+
+Configuration
+-------------
+
+In these examples, the configuration files will be at
+``<path/to/ytdl-sub/config>/config.yaml`` and
+``<path/to/ytdl-sub/config>/subscriptions.yaml``. Starting the container the first time
+will populate those files with default examples.