Success - New Multisite Install on Dedicated server using ServerPilot, Nginx and Apache

installation
8

Sorry, replied too soon and then found my answer above:

in app.yml:

expose:
  - "127.0.0.1:8888:80" # fwd host port 80 to container port 8888 (http)
  - "2222:22" # fwd host port 22 to container port 2222 (ssh)

in /etc/nginx-sp/vhosts.d/discourse.d/main.conf (renamed to main.custom.conf as suggested):

location / {
    proxy_pass $backend_protocol://$backend_host:8888;
}