[Mono.Posix] Fix Makefile to point to correct Docker image
[Mono.Posix] Fix Makefile to point to correct Docker image

The command line had an error to run a specific Image ID.  This fix
should pull the right version.

bholmes/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,bholmes/XamarinComponents,bholmes/XamarinComponents,xamarin/XamarinComponents,bholmes/XamarinComponents,bholmes/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,bholmes/XamarinComponents