Well, light, and the resulting sky glow comes in a lot of different wavelengths, so the first question you need to ask yourself is what range are you looking for? If this project is intended to give someone a good idea as to whether it is worth their while to drag out the telescope on a given night, chances are you only care about human visible wavelengths (but don't underestimate the amateur radio astronomers in your area!). My phone's camera will pick up light in the near-infrared, so you want to be careful.
Secondly, you need a camera with very low noise. If you go into a windowless room with the lights off, and take a picture, you want to have as close to 0x000000 rgb measurement from every resulting pixel as possible. A magnitude 0 star will only produce 2.08 microlux, so electrical noise can alter your results significantly.
Third, and this may be obvious, make yourself a cardboard "horizon shield" (a name I made up) to block any direct sources of light.
And if it isn't of vital importance to use an actual camera, since you are using a raspberry pi, you might just want to go with a lux meter instead. This one has a separate meter for the human visible wavelength, and the noise should be very low, depending upon how you wire it and what voltage you supply.
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