Add Dockerfile to build Debian package

[nvllsvm]

Building packages often requires multiple steps and customized environments. Having a Dockerfile to build the Debian package:

• Creates portable, reproducible build environments.
• Allows one to build the Debian package on any system.
• Documents how exactly to build the package. Our ancestor project was notoriously bad at this.

In other words - one should be able to build a Debian package doing nothing more than this. This example assumes #18 has been implemented.

$ git clone https://github.com/jellyfin/jellyfin
$ cd jellyfin/jellyfin
$ docker build -f Dockerfile.debian_package .

[joshuaboniface]

The current build procedure is just as simple and has no dependency on Docker, just on the dotnet-sdk-2.1 package and standard debuild framework - with Mono it was a nightmare, but luckily .NET Core made this a lot easier. However I don't see a reason we can't do both, as long as the Docker setup doesn't preclude doing it the old-fashioned non-Docker way.

[nvllsvm]

@joshuaboniface One of the intentions of having this in addition to the Debian build files is that it will make CI/CD integration much nicer.

[joshuaboniface]

Yup agreed on the CI integration, just wanted to be sure!