adding additional readme warning for using docker

https://gist.github.com/douglasmiranda/9de51aaba14543851ca3#file-option2-py

Author: daniel-butler

docs/installation.rst

@@ -102,3 +102,15 @@ settings module::
 You can change the logic of determining whether or not the Debug Toolbar
 should be shown with the :ref:`SHOW_TOOLBAR_CALLBACK <SHOW_TOOLBAR_CALLBACK>`
 option.  This option allows you to specify a custom function for this purpose.
+
+.. warning::
+
+    If using Docker the following will set your internal ips correctly only if you are in Debug mode.::
+    
+        if DEBUG:
+            import os  # only if you haven't already imported this
+            import socket  # only if you haven't already imported this
+            hostname, _, ips = socker.gethostbyname_ex(socket.gethostname())
+            INTERNAL_IPS = [ip[:-1] + '1' for ip in ips] + ['127.0.0.1', '10.0.2.2']
+            
+