To serve Ghost from a subfolder instead of the root of the domain, you'll need to edit /etc/nginx/sites-enabled/ghost And simply change the location block like:

server {
    listen 80 default_server;
    listen [::]:80 default_server ipv6only=on;

    server_name my-ghost-blog.com; # Replace with your domain

    root /usr/share/nginx/html;
    index index.html index.htm;

    client_max_body_size 10G;

    location /blog {
        proxy_pass http://localhost:2368;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_buffering off;
    } 
}

And then restart Nginx: service nginx restart

Next you'll also need to update Ghost's configuration file as well. Edit the URL in /var/www/ghost/config.js

config = {
    // ### Production
    // When running Ghost in the wild, use the production environment
    // Configure your URL and mail settings here
    production: {
        url: 'http://my-ghost-blog.com/blog',
        mail: {},

And restart Ghost with service ghost restart

You should be all set to go! You can now service files as normal from the Nginx document root as well.