Make prod builds but with docs

Switch to publishing prod firmware images like is recommended when
distributing firmware with Nerves. This wasn't done since the Nerves
default mix.exs strips beam files and we wanted the docs. It's possible
to strip everything but the docs, so this commit adds that option.

Author: fhunleth

.circleci/config.yml

@@ -2,6 +2,8 @@ defaults: &defaults
   working_directory: /nerves/build
   docker:
     - image: nervesproject/nerves_system_br:1.13.2
+  environment:
+    MIX_ENV: prod
 
 elixir_version: &elixir_version
   ELIXIR_VERSION: 1.11.2-otp-23

mix.exs

@@ -64,7 +64,7 @@ defmodule CircuitsQuickstart.MixProject do
       cookie: "#{@app}_cookie",
       include_erts: &Nerves.Release.erts/0,
       steps: [&Nerves.Release.init/1, :assemble],
-      strip_beams: Mix.env() == :prod
+      strip_beams: [keep: ["Docs"]]
     ]
   end
 end