title: Run `docker-php-ext-install` FROM container other than php
body: <p>I have a problem with Docker (docker-compose). I want to install some PHP extensions using <code>docker-compose.yml</code>, but I am unable to do this, because my .yml has <code>FROM ubuntu</code> and not <code>FROM php</code>. Is there any way I can achieve or access the <code>docker-php-ext-install</code>?</p>
  
  <h3>Dockerfile</h3>
  
  <pre><code>FROM ubuntu:16.04
  
  RUN apt -yqq update
  RUN apt -yqq install nginx iputils-ping
  RUN docker-php-ext-install pdo pdo_mysql mbstring
  
  WORKDIR /usr/local/src
  
  COPY docker/nginx/dev.conf /etc/nginx/conf.d/dev.conf
  COPY docker/nginx/nginx.conf /etc/nginx/nginx.conf
  
  CMD ["nginx"]
  </code></pre>
  
  <h3>docker-compose.yml</h3>
  
  <pre><code>version: "2"
  services:
    mariadb:
      image: mariadb
      environment:
        - MYSQL_ALLOW_EMPTY_PASSWORD=1
        - MYSQL_ROOT_PASSWORD=
    phpmyadmin:
      image: phpmyadmin/phpmyadmin
      ports:
        - "8080:80"
      restart: always
      environment:
        - PMA_HOST=mariadb
      links:
        - mariadb
    php:
      image: php:7.1.1-fpm
      ports:
        - "9000:9000"
      volumes:
        - .:/dogopic
      links:
        - mariadb
    nginx:
      build: .
      ports:
        - "8000:80"
      volumes:
        - .:/dogopic
      links:
        - php
  </code></pre>
  
  <h3>Console output (fragment)</h3>
  
  <pre><code>Step 5/9 : RUN docker-php-ext-install pdo pdo_mysql mbstring
   ---&gt; Running in 445f8c82883d
  /bin/sh: 1: docker-php-ext-install: not found
  </code></pre>