You’ll need to change the blog URL in Ghost to https://volleybrains.com/masterclass (url in config.production.json, and edit your Nginx config, so the location matches, i.e., location /masterclass {.
In addition, you’ll need to set redirects for existing pages and posts etc., so inbound traffic lands on the correct page. This needs to be done in Nginx. Something like this.
if ($request_uri !~ /masterclass/) {
rewrite ^ https://volleybrains.com/masterclass permanent;
}
Edit: If you are creating the landing post with Ghost, then you can use Routes to move posts and archives under /masterclass, and create the landing page in your theme. All other pages will be off the root folder. Then, redirects may be handled in Ghost, e.g., redirects.json