Yeah, your elixir Docker base image is pretty old.
Here I posted an example project that is building a release in Docker
Demo of distributed Elixir with libcluster and DNS
Lately I use the hexpm/elixir images. Here is the latest alpine image hexpm/elixir:1.10.4-erlang-23.0.3-alpine-3.12.0
About the missing variable: what do you have in your config.exs, prod.exs?
