Update nightly NuGet config

Author: lbussell

tests/Microsoft.DotNet.Docker.Tests/TestAppArtifacts/NuGet.config.nightly

@@ -5,5 +5,6 @@
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
     <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
     <add key="dotnet9" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet9/nuget/v3/index.json" />
+    <add key="dotnet10" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet10/nuget/v3/index.json" />
   </packageSources>
 </configuration>