use request.host instead of relying on possibly faulty/internal hostname

[nicolasfranck]

Socket.gethostname does not always return the correct hostname as used on the internet,
especially if the public name is just an alias.

[briri]

We need to do some testing on this one to see what impact it has on servers behind a load balancer. Our concern is that request.host may not have the hostname for the LB.

[nicolasfranck]

Shouldn't the load balancer add headers like X-Forwarded-Host to make the rails happy?
That's the way I do it. Socket.gethostname would in that case generate urls that cannot
be resolved by the browser..

[briri]

Our LB setup and admin is performed by another team so I will need to check in with them and experiment to see what we're getting for this new approach. @raycarrick-ed or @xsrust do you want to try it on your end when you have a chance to see what impact it would have for your instances?

[xsrust]

The request.host version is what we're using in our Multi-tenant customization to check the URL the user's coming in from so this shouldn't cause problems from our end.

Just adding @briri as an explicit reviewer so we can ensure you've been able to check in with your infrastructure team before we merge.

[briri]

we override that particular file, and I would imagine most installations do as well, so I think any risk is limited.
With that said, I updated my branded version of that file and the suggested change worked without issue.