Update to .NET Core 2.0

[philippgille]

Most .NET people waited for .NET Standard 2.0 and .NET Core 2.0 for starting a new project or moving an existing project from the full .NET Framework, so this template should be based on .NET Core 2.0 for others to be useful.

SCDs become a lot more useful as well, as there are less runtimes to publish for, while the SCDs will be able to run on more operating systems, see:

• New RIDs according to the .NET Core 2.0 announcement.
• More supported operating systems: .NET Core 2.0 release notes - Supported OS versions

[philippgille]

The RID catalog was recently updated, now contains the new portable RIDs and even Android: https://docs.microsoft.com/en-us/dotnet/core/rid-catalog

No Alpine yet, that's to come in .NET Core 2.1. Also see #4 for that.