Migrating from wordpress:php8.2-apache to wordpress:php8.3-fpm-alpine and nginx:stable-alpine as base image for wordpress applications

I am trying to migrate my Docker image which uses library/wordpress:php8.2-apache as base image to library/wordpress:php8.3-fpm-alpine and nginx:stable-alpine as base images. Here is my Dockerfile

# Stage 1: Build the WordPress application
FROM library/wordpress:php8.3-fpm-alpine AS build

# Install wp-cli
RUN curl -O https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar && 
    chmod +x wp-cli.phar && 
    mv wp-cli.phar /usr/local/bin/wp

# Use different mirrors
RUN sed -i -e 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/g' /etc/apk/repositories

# Install Postfix
RUN apk update && 
    apk add --no-cache postfix && 
    postconf -e myhostname=mydomain.com && 
    postconf -e relayhost="mailfwd.mydomain.com" && 
    postfix start && 
    postfix reload

# Configure PHP and create directory if it doesn't exist
RUN mkdir -p /usr/local/etc/php/conf.d/ && 
    echo "sendmail_path=sendmail -t -i" >> /usr/local/etc/php/conf.d/sendmail.ini

# Install packages: wget, bash
RUN apk add --no-cache wget bash

# Install net-tools
RUN apk add --no-cache net-tools

# Copy built files to WordPress files system
COPY default/ /var/www/html/wp-content/themes/default/

# Set permissions for the wp-install-plugins.sh script
COPY wp-install-plugins.sh /usr/local/bin/
RUN chmod +x /usr/local/bin/wp-install-plugins.sh

# Grant permissions to the www-data user
RUN chown www-data:www-data /usr/local/bin/wp-install-plugins.sh


# Stage 2: Build the NGINX server
FROM nginx:stable-alpine AS nginx-server

# Install bash in current image
RUN apk update && apk add --no-cache bash

# Check if www-data group exists, if not, add it
RUN if ! getent group www-data >/dev/null; then addgroup -g 1000 www-data; fi

# Add www-data user with existing www-data group
RUN adduser -D -H -u 1000 -G www-data -s /bin/bash www-data

# Copy the NGINX configuration
COPY nginx.conf /etc/nginx/conf.d/default.conf

# Copy default index.php file
COPY default/index.php /var/www/html/

# Copy static content from WordPress build stage
COPY --from=build --chown=www-data:www-data /var/www/html/wp-content /var/www/html/wp-content

# Set permissions for static content
RUN find /var/www/html/wp-content -type f -exec chmod 644 {} ; && 
    find /var/www/html/wp-content -type d -exec chmod 755 {} ;

# Expose port 80 to allow incoming connections
EXPOSE 80

# Command to start NGINX when the container starts
CMD ["nginx", "-g", "daemon off;"]

My nginx.conf file is:

server {
  listen 80;   # Listen on IPv4 address only
  access_log off;
  root /var/www/html;

  index index.php;
  server_tokens off;
  location / {
    # First attempt to serve request as file, then
    # as directory, then fall back to displaying a 404.
    try_files $uri $uri/ /index.php?$args;
  }
  # pass the PHP scripts to FastCGI server listening on wordpress:9000
  location ~ .php$ {
    fastcgi_split_path_info ^(.+.php)(/.+)$;
    fastcgi_pass wordpress:9000;
    fastcgi_index index.php;
    include fastcgi_params;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    fastcgi_param SCRIPT_NAME $fastcgi_script_name;
  }
}

I am able to successfully build the image and run the container and the container is healthy as well,but for some reason when I go inside the container and run any command it just shows permission denied

docker exec -it my_container bash
my_container:/# ls -l
bash: /bin/ls: Permission denied

I also see the following error when I check the logs of my container by docker logs -f my_container while trying to hit the url of my application

2024/02/29 03:06:59 [error] 21#21: *1 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.2, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://172.18.0.13:9000", host: "myapp.mydomain.com"
2024/02/29 03:06:59 [warn] 21#21: *1 upstream server temporarily disabled while connecting to upstream, client: 172.18.0.2, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://172.18.0.13:9000", host: "myapp.mydomain.com"

Is there anything wrong with my Dockerfile and the configuration? May I please know what I am doing wrong or missing so that I can fix these issues?