docs: add a guide on converting a running compute session to a new im…

…age by admins (#30)

docs/appendix/appendix.rst

@@ -69,6 +69,7 @@ model.)
 
 Alternatively, you can run the ``nvidia-smi`` command from the web terminal to query the GPU usage history inside the container.
 
+
 Automated job scheduling
 ------------------------------------------------
 
@@ -174,6 +175,46 @@ Like this, you can utilize various versions of major libraries such as
 TensorFlow and PyTorch through Backend.AI without unnecessary effort to install them.
 
 
+Convert a compute session to a new private Docker image
+-------------------------------------------------------
+
+If you want to convert a running compute session (container) to a new Docker image
+that you can use later to create a new compute session, you need to prepare your
+compute session environment and ask administrators to convert it.
+
+- First, prepare your compute session by installing the packages that you need
+  and adjust the configurations as you like.
+
+  .. note::
+
+     If you want to install OS packages, for example via ``apt`` command, it
+     usually requires the ``sudo`` privilege. Depending on the security policy
+     of the institute, you may not be allowed to use ``sudo`` inside a
+     container.
+
+     It is recommended to use :ref:`automount folder<using-automount-folder>` to
+     install :ref:`Python packages via pip<install_pip_pkg>`. However, if you
+     want to add Python packages in a new image, you should install them with
+     ``sudo pip install <package-name>`` to save them not in your home but in
+     the system directory. The contents in your home directory, usually
+     ``/home/work``, are not saved in converting a compute session to a new
+     Docker image.
+
+- When your compute session is prepared, please ask an administrator to convert
+  it to a new Docker image. You need to inform them the session name or ID and
+  your email address in the platform.
+- The administrator will convert your compute session to a new Docker image and
+  send you the full image name and tag.
+- You can manually enter the image name in the session launch dialog. The image
+  is private and not be revealed to other users
+
+  .. image:: session-creation-by-specifying-image-name.png
+     :width: 400
+     :align: center
+
+- A new compute session will be created using the new Docker image.
+
+
 Backend.AI Server Installation Guide
 -----------------------------------------