Glue, Tape or ? to attach a light weight solar panel?

Up to you but I would not stick anything to the roof. Note that you would be sticking to the membrane and that may/may not void any warranty you might have. These membranes are delicate enough as it is.

If I were you I would fasten it to a frame then apply the frame to the roof using screws (which you already have anyway). The frame would be of a big enough size to allow some growth, just in case down the road that "wee bit more" is not enough.