docs(ct-base): document startInBackground.sh IQSS#8932

doc/sphinx-guides/source/container/base-image.rst

@@ -248,6 +248,22 @@ Note: for the Github Action to be able to push to Docker Hub, two repository sec
 
 
 
+Extension Points
+++++++++++++++++
+
+The entrypoint shell script provided by this base image will by default ensure to:
+
+- Run any scripts named ``${SCRIPT_DIR}/init_*`` or in ``${SCRIPT_DIR}/init.d/*`` directory for initialization
+  **before** the application server starts.
+- Run an executable script ``${SCRIPT_DIR}/startInBackground.sh`` in the background - if present.
+- Run the application server startup scripting in foreground (``${SCRIPT_DIR}/startInForeground.sh``).
+
+If you need to create some scripting that runs in parallel under supervision of `dumb-init <https://github.com/Yelp/dumb-init>`_,
+e.g. to wait for the application to deploy before executing something, this is your point of extension: simply provide
+the ``${SCRIPT_DIR}/startInBackground.sh`` executable script with your application image.
+
+
+
 Other Hints
 +++++++++++