force containers tasks to net taskhost

[baronfel]

This is a quick test of the .NET TaskHost feature in MSBuild, not intended for actual merge.

To test this:

• install dev18
• build this branch
• launch the VS dev command prompt
• run eng/dogfood.ps1 from this repo
• clone baronfel/sdk-container-demo repo
• make sure you have docker running
• delete the global.json from that cloned repo
• do a container publish from dotnet: dotnet publish -t:PublishContainer - should work
• do a container publish from msbuild.exe: dotnet /t:Publish /t:PublishContainer - should work too!
• Take a binlog and verify the .NET Task is used.