If the Valkyrie can't hold cargo, how are the players supposed to transport small supplies (ammo, food...) and weapons (like mortars or anti-tank weapons, railguns or RPGs etc...) to the front lines?

The Valkyrie's purpose is to carry troops and a vehicle down to a location, drop them off, and provide close support via its weaponry. It is very much a dropship/gunship rather than a resupply ship. While some dropships may simply drop off the team and jet, the Valkyrie is equipped to provide continued support for drop off and extraction. The troops onboard have a gunrack per seat alongside storage lockers for additional items if the crew are not using them. The combination of troops, FPS weapon racks, vehicle deployment, and the ability to provide close support, all makes the Valkyrie a strong candidate for when you need to get in and out planetside.