Run as PUID:PGID with a configurable umask

The image now starts through docker-entrypoint.sh: as root it hands
/data and the backup folder to PUID:PGID (only files owned by someone
else, so restarts stay fast), applies UMASK, and runs Canon as that
user with su-exec. Started with --user, it runs Canon directly. The
image defaults to 1000:1000; the Compose file presets Unraid's 99:100.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -28,6 +28,11 @@ services:
       - ${CANON_BACKUP_PATH:-canon-backups}:/backups
     environment:
       CANON_BACKUP_DIR: /backups
+      # Who owns the files Canon writes. 99:100 is Unraid's nobody:users;
+      # use 1000:1000 (or your own `id -u`:`id -g`) elsewhere.
+      PUID: ${PUID:-99}
+      PGID: ${PGID:-100}
+      UMASK: ${UMASK:-022}
     env_file:
       - path: .env
         required: false