Make image more configurable

Some operators want to set more advanced tor config options like:

• BandwidthRate and BandwidthBurst
• BridgeDistribution
• ...?

We should make it possible to pass these to the docker image. Instead of predicting what options our operators would like, it would be great if we could pass arbitrary config options to the image. Once this is done, let's not forget to update our docker bridge setup guide.