How to stop Nginx from adding port to external urls when it does reverse proxy
Looking at your nginx config, it will proxy all the requests under / to http://localhost:8080. I'm unable to see from the above config snippet that it would proxied to https://res.cloudinary.com:80/.
If I have to guess, it's the application/service that running on your localhost on port 8080 that responsible to that. Have you check if that's the case?