Docker configuration with template

I'm trying to decouple a Docker image from a configuration file used by the image. The intent is to keep the image fairly generic. The issue is that the application configuration varies slightly due to clustering (there's a master as well as workers). At first I thought I could create two configs-- one for the master and one for the workers and then use a template to examine a node label and include the correct config based on the label. This doesn't seem to work. Any recommendations for conditionally including a config?