There are easier and more discreet ways to generate a 404 error to probe for server information. This URL pattern is an artifact of Fortigate/Fortinet and the modern cloud networking environment.
Fortinet's "stop watching porn at work" page makes a few calls to the above paths at
url.fortinet.net:8008 in order to download static assets.
Your site is currently hosted on an IP that once was in scope of Fortinet's IP pool. It no longer is, but some hardcoded IP, DNS cache or crawler somewhere is having a hard time coping with this fact and refuses to let go, so you're seeing traffic errantly being redirected to your site instead of being sent to Fortinet.
You'll see similar behavior when you look at the HTTP logs for any web service you start up on a public cloud compute instance-- lots of traffic intended for the previous tenant. This is not a malicious request nor indicative of an attack and is safe to ignore.
For simple but effective security make sure your webserver is checking host headers-- it should only accept traffic that references your actual domain. Anybody who tries to go to
http://184.108.40.206/index.html should not be served, they should only be served if they requested
Doing things this way you can also configure your webserver to return custom HTTP codes (gone, permanently moved, etc.) to anybody requesting resources that belonged to former tenants.