I recently worked on putting a phoenix app in a container for my side I made it work to compile without much issue in end. Also able to run some tasks without mix using a docker container. MIX_ENV is the important elixir mix var you should use to decide environment for your builds. You can have other variables that you can set for your environment to control runtime I have found it useful to call it RELEASE_LEVEL which I keep to , production, staging, test while MIX_ENV is prod, dev and test together they work well
