Add build of libuv for supporting Kestrel Web Server

As Kestrel is supposed to be the major web server backend on ASP.NET for Linux, I wanted to give it a try. So, I added build procedure of libuv to satisfy the dependency.

I created and tested my PR against 1.0.0-beta1.
tested automated build on my fork at Docker Hub: https://registry.hub.docker.com/u/muojp/aspnet-docker/builds_history/82573/ and also tested on derived https://registry.hub.docker.com/u/muojp/hellovnext-docker/ for making sure everything works fine.

For double checking, please try this:

```
$ docker run -p 5000:5000 -di muojp/hellovnext-docker:latest /bin/bash -c "kpm restore && k web"
$ ab -k -n 100 -c 20 http://localhost:5000/
-> ok
$ docker run -p 5000:5000 -di muojp/hellovnext-docker:latest /bin/bash -c "kpm restore && k kestrel"
$ ab -k -n 100 -c 20 http://localhost:5000/
-> ok
```

(Note: I've already signed CLA)