This image shows how to fasten your shaker cabinet door parts together. It was taken from a video we have on our website. When you are finally fastening the stiles and rails together, this is done by shooting short brads along the edge of the stiles. The brads are sinking into the tenon joint that was cut to fit inside of the groove in the stile piece.
The joints have yellow glue in them, but the panels are not glued. They must be left unglued and fastened to allow for expansion and contraction of the wood.
If you were to fasten the center flat panel with glue it might eventually force the joints on the stiles and frames to crack.
For more information go here: How To Build Shaker Kitchen Cabinet Doors