Customization of service (Dockerfile or install script)

[Note : restrictions on number of links or images are quite upsetting for anyone actually putting some effort into the question. Dear spammers…]

I’ve recently decided to give RancherOS a go - my question is : how to customize (ex: via custom Dockerfile) a service in RancherOS ?

Context :
• Intrigued by is coming soon I’m trying RancherOS locally in VirtualBox (Debian 8 Jessie in bridged network, so I have access to the VM at + edited my computer’s host file to test on domains *
• Service (volumes) : [google drive]/file/d/0B78zUNe7SQFeRG5BeG02Z3o0SDg/view?usp=sharing (I’ve unpacked in the RancherOS VM file drupal-8.0.1.tar.gz in /data_localhost/htdocs/ and renamed the dir as docroot)
• LoadBalancer : [google drive]/file/d/0B78zUNe7SQFeaEg3VHNtdElPWXM/view?usp=sharing
• Reached OK : [google drive]/file/d/0B78zUNe7SQFeWEMzNXdHVVFvbmc/view?usp=sharing
• Install Fail :

Need :
• Following [github]/docker-library/docs/tree/master/php I’m trying to install the missing dependencies (using docker-php-ext-install and/or docker-php-ext-configure), but when I’m adding a service in RancherOS UI, I can’t find anywhere how (or if) one can customize (without going through the hassle of installing a private docker image repo) the container, like by using a custom Dockerfile.
• Alternative : from scratch with custom old school setup script, like debian-quickup/ at master · Paulmicha/debian-quickup · GitHub

Any pointers or help greatly appreciated.
( ^∇^)