Build flashlight images on top of the Git tree

[jokesterfr]

To prevent any zip shortage here, and allow a quick build of any branch of the PrestaShop project or its forks, we would like to dive more into the packaging of PrestaShop itself.

By directly building the PrestaShop sources, more flexibility will be available on Flashlight. However, due to potentially unmaintained debian dependencies, such build could be tricky to operate. Therefore a fallback on any given zip is expected to be still working to avoid headache.