Use reverse proxy in Azure Container Apps Environment

I hope my solution will help you out.
My approach is a little bit different than yours. I am using an App service with a container as source code. The image is the nginx:latest.

Since I am using a proxy pass, I am using upstream blocks

upstream bak {
  server $BAK_HOST:$BAK_PORT;
}

The variables are been replaced by the entrypoint automagically during deployment and the variables are set on the App service configuration. You can use static host and ports in your case. The value of the variables is the FQDN nameservice of the backend in my scenario. <bak>.azurewebsites.net

I am using a map block as well, to use variables when needed, to keep it clean

map $proxy_x_server $server {
  bak $BAK_HOST;
}

Then, the actual server block

server {
  listen 80;
  server_name *.yourdomain.com;

  location /api {    
    try_files /dev/null @bak;
  }
  ... #some other locations
  location @bak {
    set $proxy_x_server "bak";
    include conf.d/bak.headers;
  }

My conf.d/bak.headers includes some of the following configurations which you will probably need as well.

proxy_cache_bypass $http_upgrade;
proxy_hide_header X-Powered-By;

proxy_pass https://$proxy_x_server;
proxy_redirect https://$server https://$host;
proxy_set_header Connection keep-alive;
proxy_set_header Host $server;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;

In the bak.headers file I am adding a bunch of headers, so in your case you probably will have to do that yourself as well.

example

add_header Access-Control-Allow-Headers "Access-Control-Allow-Origin, Origin, Content-Type, Authorization, X-Account";
add_header Access-Control-Allow-Methods "OPTIONS, POST, GET";
add_header Access-Control-Expose-Headers "WWW-Authenticate, Content-Disposition, X-Query-Token-Expired";

I hope I was helpful :D