fix(docker): keep worker task runner alive with retry loop

Original command exited after a single tasks:run pass, causing
worker container to stop instead of running scheduled tasks
continuously.

- INSTALL.md: wrap `php spark tasks:run` in a shell loop that
  reruns it every 60s, so worker container matches
  `restart: unless-stopped` intent and keeps processing tasks

Signed-off-by: objecttothis <17935339+objecttothis@users.noreply.github.com>

INSTALL.md

@@ -135,7 +135,7 @@ Add a second service to your `docker-compose.yml` pointing at the same database:
 ```yaml
 worker:
   image: opensourcepos
-  command: php spark tasks:run
+  command: sh -c "while true; do php spark tasks:run; sleep 60; done"
   depends_on:
     - db
   restart: unless-stopped