Resource interpreted as Stylesheet but transferred with MIME (localhost vs domain) - php

Question

I am running my php code in docker with Nginx and fastcgi
I am exposing port 9999 of my docker container
So to invoke my application I do:
http://localhost:9999/index.php
I also have a localdomain: example.com which is pointing to 127.0.0.1 in /etc/hosts.
So I can invoke my application using example.com:9999/index.php
Strange thing I noticed is, when I use localhost url, I am getting an error on my browser
Resource interpreted as Stylesheet but transferred with MIME type application/octet-stream: "http://localhost:9999/css/login.css?v=1".
I don't get the above error if I inovke it using example.com
Can somebody explain why is the strange behavior happening?
The nginx config is as follows:
events {
multi_accept on;
worker_connections 65535;
}
http {
# MIME
default_type application/octet-stream;
include /etc/nginx/mime.types;
# logging
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log warn;
server {
listen 80 default_server;
server_name _;
index index.php;
root /code/Public;
location / {
try_files $uri /index.php$is_args$args;
}
location ~ \.php {
try_files $uri =404;
fastcgi_split_path_info ^(.+\.php)(/.+)$;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $realpath_root$fastcgi_script_name;
fastcgi_param SCRIPT_NAME $fastcgi_script_name;
fastcgi_index index.php;
fastcgi_pass 127.0.0.1:9000;
}
}
}
This is my Dockerfile config:
FROM phpdockerio/php73-fpm
RUN apt-get update
RUN apt-get install -y php7.3-memcache
RUN apt-get install -y php7.3-curl
RUN apt-get install -y nginx
ADD docker/nginx.conf /etc/nginx/
CMD ["sh", "-c", "service nginx restart ; /usr/sbin/php-fpm7.3 -O"]
FYI: I am using slim-framework. here is the reference doc for setting up nginx