Add documentation or support for other registry images

[joseajunior]

From the documentation on how to use it, we can see the field: robot_runner_image but I don't see any type of support for images in custom registries.
If support, that should be shown on the documentation.
If doesn't supports, can you add this kind of support?

[joonvena]

It doesn't currently work with images from repositories that needs authentication. I can add this feature.