How about using pressure treated wood to build a form to the dimensions of your ramp, filling it with coarse sand to about 2" below the top and using pavers for the ramp surface. IMO, it'll look better than wood or concrete, lasting as long or longer.

I would not use concrete unless properly supported. Footer below frost line if you live in a colder climate. Concrete unsupported in a cold climate WILL settle and or shift on you, over time. If you go the concrete route, I would not anchor it to the building. Your shed most likely is also "free floating".

Luckily, I'm in central Texas, so the ground doesn't freeze. That said, I didn't attach it to the building. Time will tell how this holds up.