Runtime config improvements + add support for a .env

[joshk]

Adding the Dotenvy improves the development and deployment experience.

You can specify any environment variable used in runtime.exs in a local (and .gitignored) .env file, allowing for easy dev environment customizations. Or you can use this in production for configuring the production runtime.

One notable change is that I have removed

nerves_hub_web/config/runtime.exs

Lines 37 to 54 in 74e410e

	if System.get_env("RELEASE_MODE") do
	sync_nodes_optional =
	case System.fetch_env("SYNC_NODES_OPTIONAL") do
	{:ok, sync_nodes_optional} ->
	sync_nodes_optional
	|> String.split(" ", trim: true)
	|> Enum.map(&String.to_atom/1)
	:error ->
	[]
	end
	config :kernel,
	sync_nodes_optional: sync_nodes_optional,
	sync_nodes_timeout: 5000,
	inet_dist_listen_min: 9100,
	inet_dist_listen_max: 9155
	end

as using sys.config should be the preference. See:

• https://github.com/slashdotdash/til/blob/master/elixir/distributed-erlang-sync-nodes.md
• https://www.erlang.org/doc/man/config.html#sys.config

This can be added to the release before starting the application.

[joshk]

Closing this for the time being