Hi @dtpuser! Welcome to the community. 
In this case, I don't necessarily expect https-portal to forward the request to the api container, given that you haven't yet configured SSL for your instance. As a service, https-portal is built on top of nginx but isn't meant to function as a typical reverse proxy. It's primary purpose is to provision a self-signed certificate and handle all subsequent SSL traffic. In fact, the only reason we realistically open up port 80 is to complete Let's Encrypt's HTTP challenge.
I think you should be fine to move on with setting up a domain and configuring SSL, but want to first clarify the communications error that you're seeing. The script that generates the docker.env file makes a call to myip.opendns.com in order to fetch your public IP address, but it looks like that was blocked by a firewall or something similar. That whole block should look something like the below, except with a real IP address:
# Change '123.0.45.678' to retool.yourcompany.com to set up SSL properly
DOMAINS=123.0.45.678 -> http://api:3000
Let me know if you run into any additional issues or just have follow-up questions.