title: Docker PHP ext enable command failing
body: <p>What is required to have <code>docker-php-ext-...</code> commands available? I am creating an Alpine image</p>
  <pre><code>FROM alpine:3.12
  
  RUN apk update &amp;&amp; \
      # Add support for PHP7.4.
      apk add --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/community \
      php php7-fpm nginx composer git 
      ....
  
  # PHPIZE dependencies + xdebug
  RUN apk add --no-cache autoconf file g++ gcc libc-dev make pkgconf re2c \
      php7-pecl-xdebug \
      &amp;&amp; docker-php-ext-install xdebug
  
  ...
  </code></pre>
  <p>But I am getting <code>/bin/sh: docker-php-ext-install: not found</code>.</p>
  <p>How can I run the command?
  Also, can <code>apk add php7-pecl-xdebug</code> replace installing <code>pecl</code> as a dependency and requiring the extension that way?</p>