title: I am not able to install mbstring using docker file
body: <p>Hi i am new to docker.</p>
  <p>I have cloned my laravel code from</p>
  <p><a href="https://github.com/laravel/laravel/tree/8.x" rel="nofollow noreferrer">https://github.com/laravel/laravel/tree/8.x</a></p>
  <p>and followed the steps provided in given below link.</p>
  <p><a href="https://www.digitalocean.com/community/tutorials/how-to-set-up-laravel-nginx-and-mysql-with-docker-compose" rel="nofollow noreferrer">https://www.digitalocean.com/community/tutorials/how-to-set-up-laravel-nginx-and-mysql-with-docker-compose</a></p>
  <p>But still, mbstring is not installed in my project with php 8 version as my laravel project is compatible with a higher version than 7.2 but when i tried mbstring with 7.2 version it was easyly installed. i want to install mbstring with php 8 version.</p>
  <p>Given below is my docker file. please have a look at it.</p>
  <pre><code>FROM php:8.0.3-fpm-buster
  
  # Copy composer.lock and composer.json
  COPY composer.lock composer.json /var/www/
  
  # Set working directory
  WORKDIR /var/www
  
  # Install dependencies
  RUN apt-get update &amp;&amp; apt-get install -y \
      build-essential \
      libpng-dev \
      libjpeg62-turbo-dev \
      libfreetype6-dev \
      locales \
      zip \
      jpegoptim optipng pngquant gifsicle \
      vim \
      unzip \
      git \
      curl
  
  # Clear cache
  RUN apt-get clean &amp;&amp; rm -rf /var/lib/apt/lists/*
  
  # Install extensions
  RUN docker-php-ext-install pdo_mysql mbstring zip exif pcntl
  RUN docker-php-ext-configure gd --with-gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/ --with-png-dir=/usr/include/
  RUN docker-php-ext-install gd
  
  # Install composer
  RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
  
  # Add user for laravel application
  RUN groupadd -g 1000 www
  RUN useradd -u 1000 -ms /bin/bash -g www www
  
  # Copy existing application directory contents
  COPY . /var/www
  
  # Copy existing application directory permissions
  COPY --chown=www:www . /var/www
  
  # Change current user to www
  USER www
  
  # Expose port 9000 and start php-fpm server
  EXPOSE 9000
  CMD [&quot;php-fpm&quot;]
  </code></pre>