opening-sessions: custom jupyter image requirements

Closes #163

docs/running-notebooks/opening-sessions/index.md

@@ -14,41 +14,44 @@ It could take several minutes to start the interactive session.
 Please note that it will be automatically closed after 7 days of inactivity.
 ```
 
-!!! warning
-    REANA administrators can configure REANA to automatically close
-    inactive interactive sessions after a given period of inactivity.
-    As shown in the previous example, when you start an interactive session,
-    you will be informed about how long your inactive sessions will be kept open before they are
-    automatically closed. You can read more about this in the [closing sessions](../closing-sessions#auto-closure-of-inactive-sessions) section.
+By clicking on the link you will have access to a Jupyter notebook interface:
+
+![jupyter-notebook](../../images/interactive-session-jupyter-notebook.png){.screenshot-browser-mockup}
 
 By default, newly opened sessions will use the
-[`jupyter/scipy-notebook:notebook-6.4.5`](https://hub.docker.com/layers/jupyter/scipy-notebook/notebook-6.4.5/images/sha256-b6a4ce777b837496d5612b7ce4efba9aa015576cb6993817721b8d293a7c2a3c?context=explore) Docker image to spawn your notebook. You can supply
-your desired image by passing it to the previous command with the
+[`jupyter/scipy-notebook:notebook-6.4.5`](https://hub.docker.com/layers/jupyter/scipy-notebook/notebook-6.4.5/images/sha256-b6a4ce777b837496d5612b7ce4efba9aa015576cb6993817721b8d293a7c2a3c?context=explore)
+Docker image to spawn your notebook.
+If you would like to use a different image, you can pass it to the previous command with the
 `-i/--image` option:
 
 ```console
 $ reana-client open -w jupyter --image jupyter/scipy-notebook:notebook-6.4.7
 ==> SUCCESS: Interactive session opened successfully
 https://reana.cern.ch/f8be55e4-5d18-43f9-b977-f773fdcab163?token=<your-reana-token>
 It could take several minutes to start the interactive session.
+Please note that it will be automatically closed after 7 days of inactivity.
 ```
 
-By clicking on the link you will have access to a Jupyter notebook interface:
-
-![jupyter-notebook](../../images/interactive-session-jupyter-notebook.png)
+If you want to use a custom image, please note that it has to be based on one of the
+[official Jupyter images](https://jupyter-docker-stacks.readthedocs.io/en/latest/using/selecting.html#jupyter-base-notebook)
+(in this way, it will have Jupyter running on the port `8888` and the
+[`start-notebook.sh`](https://github.com/jupyter/docker-stacks/blob/main/base-notebook/start-notebook.sh) script available).
 
-!!! note
-    Keep in mind that this operation might take a minute, if you see
-    "Service unavailable", just wait and retry.
+!!! warning
+    REANA administrators can configure REANA to automatically close
+    inactive interactive sessions after a given period of inactivity.
+    As shown in the previous examples, when you start an interactive session,
+    you will be informed about how long your inactive sessions will be kept open before they are
+    automatically closed. You can read more about this in the [closing sessions](../closing-sessions#auto-closure-of-inactive-sessions) section.
 
 ### Open from web interface
 
 Alternatively, you can also open Jupyter notebook sessions from the
-REANA web interface by going through a list of your workflows and
-clicking on the vertical ellipsis menu on the right-hand-side and
+REANA web interface by going through a list of your workflows,
+clicking on the vertical ellipsis menu on the right-hand-side, and
 selecting "Open Jupyter Notebook":
 
-![ui-open-session](../../images/ui-open-session.png)
+![ui-open-session](../../images/ui-open-session.png){.screenshot-browser-mockup}
 
 When a workflow has an associated Jupyter notebook session opened, a
 Jupyter icon will appear next to the workflow name; you can then click