Allow custom domain extensions

[superbiche]

Hi,
In my team, we now use a .docker custom domain extension.
This, using Docker, Docker Compose & Traefik, plus a small custom script in order for XDebug to reach the remote, allows us to work with URLs like https://myproject.docker/.

Unfortunately, this extension doesn't work properly with this kind of domain: the cookie is never set.
I'm not sure if this is a limitation of this extension or a Chrome one, but whenever I try with Postman - using XDEBUG_SESSION_START=1 query param - Xdebug is working properly.
I can also add cookies to these custom domains in Postman, so I guess it's a validation issue in this extension.

Would someone be kind enough to tell me where to start investigating and I'd happily submit a PR to fix this behavior (if you agree it needs to be fixed).

Cheers,