update csharp, fsharp to .net core 3.0

[Arukim]

• dockerfile with support of both 2.1 and 3.0
• project definitions updated
• build script updated

[kuviman]

Okay now. Looks like this is almost working. There is also a new version of .net core now (3.1). It is also new LTS, so your Dockerfile needed some changes.

Current master is used on the server now. You can use .net core 3.1 when submitting with zip (specify netcoreapp3.1 in csproj). 3.0 is also installed, but doesnt work, it looks like.

So, 2.1 and 3.1 seem to work fine, but 3.0 tries to access nuget index for some reason, hence compilation fails since we don't allow internet usage when compiling.