Where to Store Container Configuration
we do this with .conf files and loading them via compose

env_file: /path/to/our.conf

they can also be used with 

docker run --env-file /path/to/our.conf ourcontainer:version

In some cases we use the variables defined in our conf files directly, meaning the code we are running reads them direct, in others we need to modify other files on a per environment basis so we use sed or other tools to make changes to a file we have turned into a template