LED security light adding a switch
You have two sets of wires to extend.  


The solar panel wires, I assume you will want to have the cable exit at the door end of the container, so you don't have a potentially leaky hole in the side. (NEVER put a hole in the roof). That needs 2 wires.  
The switch wires.  The switch has 3 modes: Off, always-on, and motion sensor.  Off needs no wires, but the others need 1 each plus a common. That's 3. Noting there only 2 wires, clearly it's using the light's metal chassis as a "common", much like is done in automobile wiring.  


Look carefully at the solar panel wiring.  If one of its wires is also attached to the machine chassis, that'd be good news, it means that wire can be shared with the chassis-side of the switch, and we can do all this with common-as-dirt 14/3 UF cable.  Otherwise you'd need exotic 14/4, or dual 14/2.

This is a low voltage device, so there's no particular risk, other than destroying the device. 

14/2 or 14/3 UF cable comes with a bare wire for safety earthing. Since we're not using it for dangerous mains voltage, we'll use it for common/chassis.  

For the switch, I would not use a mains-voltage 3-way (2-way) switch. I would use an electronics store rocker or toggle SPDT switch, with "chassis" on common and each of the other two wires on the ends. 

These devices will not like having a balky, inflexible #14 wire attached to them. Have the last 1' or so going to the device use a smaller wire like a #20 stranded wire, and tie it to the #14 with a wire nut, probably orange.