未捕获的错误:在docker容器中找不到类"Memcached"

我正在尝试将php7 slim应用程序部署到docker中.这是我的Dockerfile的样子:

I am trying to deploy php7 slim application into docker. This is how my Dockerfile looks like:

FROM php:7.1.2-apache

RUN php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" && \
    php -r "if (hash_file('SHA384', 'composer-setup.php') === '544e09ee996cdf60ece3804abc52599c22b1f40f4323403c44d44fdfdd586475ca9813a858088ffbc1f233e9b180f061') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;" && \
    php composer-setup.php && \
    php -r "unlink('composer-setup.php');" && \
    mv composer.phar /usr/local/bin/composer


ADD sourcecode  /var/www/html

RUN composer install
RUN sed -i 's/DocumentRoot.*$/DocumentRoot \/var\/www\/html\/src/' /etc/apache2/sites-enabled/000-default.conf

这是Memcached初始化的代码片段

This is snippet of code where Memcached initialized

<?php
global $config;

$cache_settings = $config['settings']['memcached'];

$memcached = new Memcached;
$memcached->addServer($cache_settings['host'], $cache_settings['port']) or die ("CANNOT connect to memcached");

我也有Memcached和Mysql容器正在运行.所有人员正在通过docker-compose文件启动.但是，当我请求我的应用页面时，会出现此错误:

I also have Memcached and Mysql containers are running. All staff is launching by docker-compose file. But when I am requesting for a page of my app, I get this error:

未捕获的错误:在...中找不到类'Memcached'

Uncaught Error: Class 'Memcached' not found in ...

如何解决此问题?