Document the differences with Intel's docker images

[sbellem]

Some questions to guide the documentation:

• How different are these images from the ones that can be built from the linux-sgx repo?
• Why use these images instead of Intel's images?
• Can you give some examples on how I could use these images?
• Do I need to build the images myself, or can I pull them from Dockerhub?
• Can I use one of these images as a base image for my own image? If yes, how?

[relyt29]

I think ease of use is a big one, even the linux-sgx repo is a pain in the ass to set up