Allow reader-only release selection in Compose

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: