It took me quite a while to get this all right. Put the following into your nginx.conf. Adjust paths as needed.

server {
  root   /Users/USERNAME/development/9ewiki;
  listen       8080;
  server_name  9ewiki;
  server_name_in_redirect off;

  location ~ \.php$ {
    fastcgi_pass   127.0.0.1:8888;
    fastcgi_index  doku.php;
    
    include        fastcgi.conf;
  }

  location / {
    root   /Users/USERNAME/development/9ewiki;
    index  index.html doku.php index.php;

    rewrite  ^/_media/(.*)              /lib/exe/fetch.php?media=$1 last;
    rewrite  ^/_detail/(.*)             /lib/exe/detail.php?media=$1 last;
    rewrite  ^/_export/([^/]+)/(.*)     /doku.php?do=export_$1&id=$2 last;
    rewrite  ^/$                        /doku.php last;


    if (-f $request_filename) { break; }
    if (-d $request_filename) { break; }
    rewrite /(.*)  /doku.php?id=$1  last;
    
    rewrite ^index.php$ /doku.php;
  }
}