roundcube custombuild & composer

[shanti]

Hello

please implement composer-action in roundcubes-custombuild:

• 
  therefore custombuild should consider templates from custom/roundcube/composer.json

• 
  furthermore a postbuild command like '$composer update --no-dev -o --apcu-autoloader' should be run postbuild in '/var/www/html/roundcube'

thanks for considering
-c-