title: Using Docker, how do you execute PHP composer commands?
body: <p>I'm new to Docker and got my PHP environment set up with this as my Dockerfile:</p>
  
  <pre><code>FROM php:7.0-apache
  COPY src/ /var/www/html/
  </code></pre>
  
  <p>Now I want to use PHP code I found on github and it says to install it I simply have to do:</p>
  
  <pre><code>composer require league/oauth2-client
  </code></pre>
  
  <p>Upon getting a shell into my docker container composer isn't even installed. How do I add composer into the Dockerfile and then execute the composer commands I need?</p>