If it's on skids, do you intend to move it around? If so the need for a permanent floor is confusing.
I know nothing about the needs of goats, but I built a 5m square roofed shelter for my sheep on grass. Covered the floor with straw, and very quickly the earth was trampled down and the grass disappeared. What is there now is a hard packed, dry smooth floor that can be cleaned out and disinfected with lime very easily.