Nginx not accepting OPTIONS requests

Using Nginx-stable image. All is good except:

I issue an OPTIONS request to server root and Nginx server responds with 405 not allowed error. It doesn’t get as far as the PHP app. There is a route for OPTIONS [/] in the PHP app (A Slim V4 routed app if it makes a difference)

I can issue a GET request to the same url and the PHP app behind it will reply. I can issue an OPTIONS requests to other endpoints served by routes and the PHP app responds as expected.

Looking through the devilbox docs for Nginx I can’t spot anything obvious. I’ve managed to jump onto the nginx box via ssh and looked at the vhost file and cannot see anything that might block the OPTIONS request.

Any ideas anyone?