Allow reader-only release selection in Compose

docs/reading-limiter.md

@@ -11,8 +11,11 @@ DRM. There is no schedule-changing API or UI.
 
 ## Docker configuration
 
-Set these deployment variables on the reader service, then recreate **only the
-reader**. Leaving `READING_WINDOWS_JSON` empty disables the limiter. Day keys
+Set these deployment variables in the Compose `.env` file next to
+`docker-compose.yml`, then recreate **only the reader**. The `reader` service
+passes them through to the container. `READER_IMAGE` selects a reader-only
+release while `APP_IMAGE` continues to select the scraper and Sentinel image.
+Leaving `READING_WINDOWS_JSON` empty disables the limiter. Day keys
 are `0` Sunday through `6` Saturday. Each day has any number of `[start,end]`
 intervals; an end earlier than its start carries into the next day. `24:00` is
 valid only as an end. Intervals are start-inclusive and end-exclusive, resolved