Add Dockerfile to build Debian package about jellyfin

[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]

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.

[joshuaboniface]

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