Our Cloud Container product is built on top of a technology called Docker that utilises containers to run websites or applications in. These containers each use an 'image' that the container runs, for example your first container may use the Apache + PHP 7.0 image and your next may use the Nginx + PHP 5.5 image.
We provide a few images out of the box, but if you need something we don't provide, that's where custom images come in. They allow you to build an image to your exact specifications and then run containers on your server using that image.
Good question. We have tried to make this product as flexible as possible and hope that you'll be able to build whatever you may need. We already have people using custom images to run Ruby on Rails, unique PHP modules and completely custom web server stacks. It's really up to you.
That's not to say there aren't any limitations, you may find some, but if you do we'll do our best to help you through them.