title: Make a global variable od "extra_hosts" in docker-compose
body: <p>Lets suppose that I have got a ten services in docker-compose and every this service need the same extra_hosts with a four records.
  I would like to define extra_hosts once and only include it to every service.</p>
  
  <p>Is it possible?</p>
  
  <pre><code>version: '3.7'
  services:
  
    web:
      build:
        context: ./apache
        dockerfile: dockerfile_apache2
      image: debian:latest
      container_name: hsthttp1
      extra_hosts:
       - "somehost1:162.242.195.82"
       - "somehost2:162.242.195.83"
       - "somehost3:162.242.195.84"
       - "somehost4:162.242.195.85"
    web2:
      build:
        context: ./apache
        dockerfile: dockerfile_apache2
      image: debian:latest
      container_name: hsthttp2
      extra_hosts:
       - "somehost1:162.242.195.82"
       - "somehost2:162.242.195.83"
       - "somehost3:162.242.195.84"
       - "somehost4:162.242.195.85"
    web3:
      build:
        context: ./apache
        dockerfile: dockerfile_apache2
      image: debian:latest
      container_name: hsthttp3
      extra_hosts:
       - "somehost1:162.242.195.82"
       - "somehost2:162.242.195.83"
       - "somehost3:162.242.195.84"
       - "somehost4:162.242.195.85"
  </code></pre>