When I build your container, this is what I see:
Step 8/10 : RUN mix deps.get
---> Running in 38022be2ebe1
** (Mix.Config.LoadError) could not load config config/config.exs
** (SyntaxError) config/config.exs:24: syntax error before: config
(elixir) lib/code.ex:192: Code.eval_string/3
(mix) lib/mix/config.ex:188: Mix.Config.read!/2
(mix) lib/mix/tasks/loadconfig.ex:37: Mix.Tasks.Loadconfig.load/1
(mix) lib/mix/tasks/loadconfig.ex:27: Mix.Tasks.Loadconfig.run/1
(mix) lib/mix/task.ex:314: Mix.Task.run_task/3
(mix) lib/mix/cli.ex:79: Mix.CLI.run_task/2
So my assumption is, that you happily changed around in your Dockerfile, but never rebuilt the container.
