Migrate common ENV vars to etc/container.env [0.17.2]

[MickMake]

Make use of the new etc/container.env mechanism to import variables into containers. Some variables will be dynamic - EG: BOX_VERSION

  "PACKAGE_VERSION": "7.2.6",
  "PACKAGE_URL": "http://php.net/get/php-7.2.6.tar.gz/from/this/mirror",
  "PACKAGE_ROOT": "/",
  "WPLIB_USER": "vagrant",
  "WPLIB_UID": "1000",
  "WPLIB_GROUP": "vagrant",
  "WPLIB_GID": "1000",
  "HOME": "/home/vagrant",
  "WORDPRESS_DB_COLLATE": "utf8mb4_unicode_ci",
  "WORDPRESS_DB_CHARSET": "utf8mb4",
  "WORDPRESS_DB_PORT": "3306",
  "WORDPRESS_DB_HOST": "172.42.0.1",
  "WORDPRESS_DB_PASSWORD": "wordpress",
  "WORDPRESS_DB_USER": "wordpress",
  "TABLE_PREFIX": "wp_",
  "WP_DEBUG": "1",
  "DB_PORT": "3306",
  "DB_HOST": "172.42.0.1",
  "DB_PASSWORD": "wordpress",
  "DB_USER": "wordpress",
  "DB_NAME": "wordpress",
  "WP_HOST": "wplib-box",
  "WP_ENV_SUPPORT": "1",
  "WPLIB_BOX": "1",
  "USER": "vagrant",
  "SERVER_NAME": "wplib.box",
  "SERVER_PORT": "80",
  "SERVER_ADDR": "172.42.0.3",