Provide documentation and building steps for unikernel examples

[cmainas]

Before deploying any unikernel over urunc it is necessary to build it. Building a unikernel is not always an easy task. As a result, we should provide some documentation and some Dockerfiles to help people build a few unikernel examples that can be used over urunc. Some of these examples are:

• Redis
• Nginx
• Httpreply

As soon as we have some documentation of building such applications with the supported unikernel frameworks, we can even create workflows that can produce such images.