Adding Pages URL to the configuration causes it to fail


I am busy preparing a demo of Gitlab for a team and I am using the current gitlab-ce docker container running one RHEL7.4

I have added a URL to my gitlab instance that is structured like this

I wanted to add a URL for my pages of

Following the documentation I added the pages URL to the pages_external_url config setting in the gitlab.rb file.
I then restarted the gitlab docker container and then the main gitlab page stops working.
if I comment out the setting again and restart everything works as expected.

My question is.
Is it possible to use pages in this manner?
Have I been a newbie and missed a setting somewhere?
How can I debug why this is failing?

Thanks for the help