Force the Celery workers to run only one task at a time

This forces the tasks to be run serially.

module_build_service/__init__.py

@@ -55,6 +55,8 @@ celery_configs = {
     name[7:]: getattr(conf, name)
     for name in dir(conf) if name.startswith("celery_")
 }
+# Only allow a single process so that tasks are always serial per worker
+celery_configs["worker_concurrency"] = 1
 celery_app.conf.update(**celery_configs)