Adding runtime config via Application.put_env/2 in Application start/2 callback

application.ex

defmodule Sample.Application do
  use Application

  # ...
  def start(_type, _args) do
    import Supervisor.Spec
    # ...
    
    # Update application environment from system environment variable when application starts
    Application.put_env(:sample_app, :configurable_env_var, System.get_env("CONFIGURABLE_ENV_VAR"))

    # ...
    Supervisor.start_link(children, opts)
  end
end