docs: Add section on how-to run celery tasks manually

Signed-off-by: Fabian P. Schmidt <kerel@mailbox.org>

docs/developer-guide.rst

@@ -80,6 +80,27 @@ In order to simulate an heartbeat of the stations 7, 23 and 42, the following co
   $ docker-compose exec web django-admin update_station_last_seen 7 23 42
 
 
+Manually run a celery tasks
+---------------------------
+
+The following procedure can be used to manually run celery tasks in the local (docker-based) development environment:
+
+- Setup local dev env (docker).
+- Start django shell
+  ```
+  docker-compose exec web django-admin shell
+  ```
+- Run an asnyc task and check if it succeeded.
+  ```
+  > from network.base.tasks import update_all_tle
+  > task = update_all_tle.delay()
+  > assert(task.ready())
+  ```
+- (optional) Check the celery log for the task output:
+  ```
+  docker-compose logs celery
+  ```
+
 Coding Style
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