what adhesive do I use to reattach soap dish in ceramic shower?

I would use "Araldite Rapid Ceramic and Glass Glue". It comes in a pack containing two tubes. You mix them together in equal parts. (Look it up on Ebay, but should be available at the big stores) Obviously follow the instructions. Do make sure that all surfaces are really clean and any old glue removed.
This will probably work OK, but my preference is always to screw things to the wall. Your dish may not be designed for this. I would prefer to get one that allowed this.
Either way run a clear mastic seal around the soap dish on completion. Good luck !

Frank
October 2011