Allow reader-only release selection in Compose

.env.example

@@ -1,4 +1,6 @@
 APP_IMAGE=mangacircus:local
+# Optional reader-only release. Leave empty to use APP_IMAGE.
+# READER_IMAGE=mangacircus:local
 VCS_REF=development
 READER_BIND=127.0.0.1
 READER_PORT=8080

docker-compose.yml

@@ -23,6 +23,7 @@ x-app: &app
 services:
   reader:
     <<: *app
+    image: ${READER_IMAGE:-${APP_IMAGE:-mangacircus:local}}
     command: [node, runtime/launch.js, reader]
     ports: ["${READER_BIND:-127.0.0.1}:${READER_PORT:-8080}:8080"]
     volumes:

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
+Set these deployment variables in the Compose `.env` file next to
-reader**. Leaving `READING_WINDOWS_JSON` empty disables the limiter. Day keys
+`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