Update composer configuration in config file

[damien-carcel]

Depending on composer version, and maybe Linux (or other) system, the composer folder that contains the cache and GitHub authentication is not at the same place: old one is ~/.composer, new one is ~.config/composer.

Another problem is with people that do not have composer (or even PHP) on there host machine. They need to be able to share there SSH key with the container if they want to install Akeneo EE.

• Composer home inside the container should be fixed with environment variable COMPOSER_HOME=/home/docker/.composer
• Install SSH client in PHP image (done in #228)
• Add a new volume in compose examples for sharing SSH keys.
• All of this has to be properly explained in the compose examples and the related documentation:
  • looking for the right composerlocation: https://getcomposer.org/doc/03-cli.md#composer-home
  • why mapping composer
  • why mapping SSH and when (optional, only for EE standard edition)