Decoupling capacitor across Raspberry Pi GPIO input (+ Arduino + robotics)
A bypass capacitor on the input to the RPi might be enough, but it depends on the duration and magnitude of the dips caused by servos.

I would use an oscilloscope to investigate the dips. If they are very short (us) then I would add arbitrary caps (100uF, 1000uF) and re-test.

Another solution would be to try and confine those peak draw pulses by adding a diode from the battery to the Arduino, then connecting the Vin to the 5V regulator to before the diode. This way, transients won't pull from the input caps of Arduino, only from the battery.