Nginx local file first, if not found use proxy as fallback / failover

location / {
    root /var/www/html;
    try_files $uri $uri/ @proxy;
}

location @proxy {
    proxy_pass https://anotherserver.com;
    proxy_set_header Host anotherserver.com;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
}