docker: Improve swh-scheduler-schedule-recurrent service startup

In order for the origin loading tasks resulting from a listing to be
successfully scheduled by that service, the loading task types must
have been registered in the scheduler database prior the execution of
the main command of that service.

So make the service depends on the swh-loader one and use a Python script
to check the celery worker used to execute loading tasks have been started.

docker/docker-compose.yml

@@ -232,6 +232,7 @@ services:
     command: schedule-recurrent
     depends_on:
       - swh-scheduler
+      - swh-loader
       - amqp
     volumes:
       - "./conf/scheduler.yml:/scheduler.yml:ro"

docker/services/swh-scheduler-worker/entrypoint.sh

@@ -16,8 +16,19 @@ case "$1" in
         wait_pgsql
 
         wait-for-it swh-scheduler:5008 -s --timeout=0
+        wait-for-it amqp:5672 -s --timeout=0
 
-        echo "Starting the swh-scheduler $1"
-        exec wait-for-it amqp:5672 -s --timeout=0 -- swh --log-level ${LOGLEVEL} scheduler -C /scheduler.yml $@
+        echo "Waiting for loader task types to be registered in scheduler db"
+        until python3 -c "
+from celery import Celery
+app = Celery('swh', broker='amqp://guest:guest@amqp/')
+assert any(worker_name.startswith('loader@')
+           for worker_name in app.control.inspect().active())" 2>/dev/null
+        do
+            sleep 1
+        done
+
+        echo "Starting swh scheduler $1"
+        exec swh --log-level ${LOGLEVEL} scheduler -C /scheduler.yml $@
         ;;
 esac