The idea being that if mycookie is not 0 (which should cover it not
being set), it is set to 1. I then added cookie_MYCOOKIE to the bypass
and no cache parameters so that if it is set to 1, they are triggered.
The backend sets mycookie to 0 if it is 1 so that on subsequent
requests, the resource is served from the cache.
However unfortunately, the "add_header Cookie: "mycookie=1";" line does
not seem to be called at all.