Nginx behind ELB, forward based on forwarding headers (host)

Question

We have an Amazon Web Services (AWS) Classic Elastic Load Balancer (ELB) that is in front of an Nginx. The Nginx is in front of a Jenkins and an EC2 Container Service Repository (ECR).

The setup is rather complex and a lot of it is due to AWS and project limitations (we're trying to host a Jenkins instance as a container in Europe => AWS ECS service => can't use ALBs or NLBs due to their limitations regarding forwarding HTTP and TCP => have to use Classic ELB; the Nginx is in there because the ECS repository can't have a custom domain name).

The Classic ELB can't filter traffic based on the domain name, so I'd like to do it in the Nginx configuration.

Can I do this in Nginx, based on the forwarding headers received from the ELB? Side question, is there some actual documentation for these headers? I found bits and pieces (for example $http_x_forwarded_proto) but nothing in the official documentation.

The current config looks a bit like this (it's an artifact from when Nginx would just do HTTP -> HTTPS and forwarding for HTTPS for the ECR URL; at the time there was an ALB in front that did the domain filtering):

server {
    listen 80;
    server_name  localhost;

    # Redirect HTTP to HTTPS.
    if ($http_x_forwarded_proto = "http") {
            return 301 https://$host$request_uri;
    }

    # Forward HTTPS requests to the Elastic Container Repository (ECR).
    location / {
            proxy_pass              "https://ecr-url";
            proxy_set_header        Host "ecr-url";
            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;
            proxy_read_timeout  90;
    }

}

I know that if is evil but I couldn't find a simpler alternative and I tried to stay within the "safe" limits of Nginx if.