Docker / Symfony / MongoDB - 无法使用 PHP 5.6 FPM 安装 mongodb

我无法使用 php:5.6-fpm 图像安装 mongodb 扩展。我的 Dockerfile 配置有什么问题？

FROM php:5.6-fpm

RUN apt-get update \
    && mkdir -p /usr/share/man/man1 \
    && mkdir -p /usr/share/man/man7 \
    && apt-get install -y --no-install-recommends vim curl debconf subversion git apt-transport-https apt-utils \
    build-essential locales acl mailutils wget zip unzip htop vim \
    gnupg gnupg1 gnupg2 \
    libmemcached-dev zlib1g-dev \
    libcurl4-openssl-dev pkg-config libssl-dev libicu-dev g++

RUN docker-php-ext-configure intl
RUN docker-php-ext-install pdo pdo_mysql zip intl

RUN pecl install mongodb
RUN docker-php-ext-enable mongodb

COPY php.ini /usr/local/etc/php/conf.d/php.ini

...
...

执行时出现以下错误 docker-compose build

Step 5/18 : RUN pecl install mongodb
 ---> Running in 5cd13b1b969c
WARNING: channel "pecl.php.net" has updated its protocols, use "pecl channel-update pecl.php.net" to update
pecl/mongodb requires PHP (version >= 7.0.0, version <= 7.99.99), installed version is 5.6.40
No valid packages found
install failed