Add option to override displayed url

This commit introduces a new alias `custom_display_url` to override the
URL info displayed at launch with a custom string.

It is intended to be used when the app is run in an environment where
the url to display to the users is not detectable reliably (proxified or
containerized setups for example).

notebook/notebookapp.py

@@ -666,6 +666,19 @@ class NotebookApp(JupyterApp):
             value = u''
         return value
 
+    custom_display_url = Unicode(u'', config=True,
+        help=_("""Override URL shown to users.
+
+        Replace actual URL, including protocol, address, port and base URL,
+        with the given value when displaying URL to the users. Do not change
+        the actual connection URL. If authentication token is enabled, the
+        token is added to the custom URL automatically.
+
+        This option is intended to be used when the URL to display to the user
+        cannot be determined reliably by the Jupyter notebook server (proxified
+        or containerized setups for example).""")
+    )
+
     port = Integer(8888, config=True,
         help=_("The port the notebook server will listen on.")
     )
@@ -1339,11 +1352,16 @@ class NotebookApp(JupyterApp):
     
     @property
     def display_url(self):
-        if self.ip in ('', '0.0.0.0'):
-            ip = socket.gethostname()
+        if self.custom_display_url:
+            url = self.custom_display_url
+            if not url.endswith('/'):
+                url += '/'
         else:
-            ip = self.ip
-        url = self._url(ip)
+            if self.ip in ('', '0.0.0.0'):
+                ip = socket.gethostname()
+            else:
+                ip = self.ip
+            url = self._url(ip)
         if self.token:
             # Don't log full token if it came from config
             token = self.token if self._token_generated else '...'