fix Dockerfile, download online sources, install build dependencies, generate SRPM, ...

Hi,

I tried to use docker-rpm-builder, unfortunately I has some issues:
* Dockerfile reference file outside this repo
* Build dependencies not fetched
* rpmbuild not using /docker-rpm-build-root

I make it works with this Pull Request.

Moreover, I added some features that I think would be interesting for the project:
* Download SourceXXX ou PatchXX if provided as an url in the spec file
* Generate SRPM
* Create a Dockerfile based on an official Centos