# config/config.exs

# ...

use Mix.Config

import_config_exist = fn filename ->
  path = Path.join("config", filename)

  if File.exists?(path) do
    import_config filename
  end
end

import_config_exist.("env.local.exs")

config :app, App.Repo,
  username: System.get_env("DB_USERNAME"),
  password: System.get_env("DB_PASSWORD"),
  database: System.get_env("DB_NAME"),
  hostname: System.get_env("DB_HOSTNAME"),
  pool_size: 10

# ...