Hi
A number of options come to mind.
1) A piece of cable fed through a hole at the other end of the container. Use the towing vehicle to pull the trailer into the container.No electrics or winch required. Only cost a bit of cable.
2) Boat winch at end of container. Winding handle through side of container.No electrics required.
3) Electric winch at end of container. Control and power cable plugs into side of container. Power taken from tow vehicle.4) Tow hitch on front of tow vehicle. Just drive th sucker in. To take out again just back up to trailer, hook up, drive off.
5) Note that if using winch as in 2 and 3 you could lift end of container so that trailer can be winched out (i.e. gravity)George