Try to configure: /* => JS UI and /api/* => to Rails API.

default.config

server {
    server_name  sourcerio.com www.sourcerio.com;
    listen       80;
    listen       443 ssl http2;
    include      letsencrypt.conf;

    client_max_body_size 10M;
    keepalive_timeout 10;

    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_pass_header Set-Cookie;

    ssl_certificate_key /etc/nginx/certs/live/sourcerio.com/privkey.pem;
    ssl_certificate     /etc/nginx/certs/live/sourcerio.com/fullchain.pem;

    location /api {
        if ($scheme != "https") {
            rewrite ^ https://$host$uri permanent;
        }

        proxy_pass   http://api:3000;
    }

    location / {
        if ($scheme != "https") {
            rewrite ^ https://$host$uri permanent;
        }

        proxy_pass   http://ui:80;
    }
}