Add .NET Core environment/build support to the docker image

See a recent build job like:

https://travis-ci.org/apache/thrift/jobs/193507709

thrift 1.0.0-dev
Building Plugin Support ...... : no
Building C++ Library ......... : yes
Building C (GLib) Library .... : yes
Building Java Library ........ : yes
Building C# Library .......... : yes
Building .NET Core Library ... : no
Building Python Library ...... : yes
Building Ruby Library ........ : yes
Building Haxe Library ........ : yes
Building Haskell Library ..... : yes
Building Perl Library ........ : yes
Building PHP Library ......... : yes
Building Dart Library ........ : yes
Building Erlang Library ...... : yes
Building Go Library .......... : yes
Building D Library ........... : yes
Building NodeJS Library ...... : yes
Building Lua Library ......... : yes

Plug-in support is disabled by default with THRIFT-4033, but I noticed we don't have .NET Core build support in the docker image.