set_secure_random_alphanum doesn't work well with fastcgi_cache

I'm trying to use a secure nonce to allow some inline code through a CSP. However, this doesn't work when combined with fastcgi_cache as the nonce value used in the web page is cached, but the nonce value output in the header is created anew.

The server config looks something like this:

	fastcgi_param CSP_NONCE $nonce;
	more_set_headers "Content-Security-Policy: style-src 'nonce-$nonce';";

	location = / {
		set_secure_random_alphanum $nonce 10;
		fastcgi_pass php;
	}

The result is that a nonce is generated and inserted into the CSP header and passed to PHP which inserts it into the HTML. But, as mentioned, when using fastcgi_cache, the header/nonce is not cached along with the page.

Start with the reported nginx configuration using set_secure_random_alphanum, fastcgi_cache, fastcgi_param, and more_set_headers. Investigate how the nonce is generated and how cached responses handle the page, header, and FastCGI parameter; done means the nonce remains consistent between the cached HTML and CSP header.