Use Cake to build .Net Core project using Debug configuration

I seem to be unable to persuade Cake to build my .Net core project in Debug configuration.

At the moment I have this:

void BuildProject(string projectPath, string configuration)
{
    var settings = new DotNetCoreBuildSettings();
    settings.Configuration = "Debug";

    DotNetCoreBuild(projectPath, settings);
}

...

Task("Build")
   .IsDependentOn("RestorePackages")
   .Does(() => 
   {
       BuildProject("src/DlnaCore.Server/DlnaCore.csproj", configuration);
   });

which just builds the release config.

Having scoured what I can find in the docs I cannot seem to find the correct incantation :-/