title: PHP Docker container exits for no reason
body: <p>I'm relatively new to docker and docker-compose so I made this file</p>
  
  <pre><code>version: '3'
  services:
    web:
      image: nginx:latest
      ports:
        - "8050:80"
      volumes:
        - ./code:/code
        - ./site.conf:/etc/nginx/conf.d/default.conf
      links:
        - php
    php:
      image: php:7.3-fpm-alpine3.9
      command: apk --update add php7-mysqli
      volumes:
        - ./code:/code
    db:
      image: mysql
      command: --default-authentication-plugin=mysql_native_password
      restart: always
      ports:
        - 3306:3306
      environment:
        MYSQL_ROOT_PASSWORD: example
  
    adminer:
      image: adminer
      restart: always
      ports:
        - 8080:8080
  </code></pre>
  
  <p>For some reason the line</p>
  
  <pre><code>command: apk --update add php7-mysqli
  </code></pre>
  
  <p>Stops php container for no reason, just prints <code>dock_php_1 exited with code 0</code></p>
  
  <p>Thus my web container also stops and service doesn't work</p>
  
  <p>What could be core of the problem and how to fix it?</p>