Make configs consistent

README.md

@@ -206,9 +206,6 @@ The following environment variables configure Livebook:
   * LIVEBOOK_SHUTDOWN_ENABLED - controls if a shutdown button should be shown
     in the homepage. Set it to "true" to enable it.
 
-  * LIVEBOOK_WITHIN_IFRAME - controls if the application is running inside an
-    iframe. Set it to "true" to enable it.
-
   * LIVEBOOK_TOKEN_ENABLED - controls whether token authentication is enabled.
     Enabled by default unless `LIVEBOOK_PASSWORD` is set. Set it to "false" to
     disable it.
@@ -216,6 +213,10 @@ The following environment variables configure Livebook:
   * LIVEBOOK_UPDATE_INSTRUCTIONS_URL - sets the URL to direct the user to for
     updating Livebook when a new version becomes available.
 
+  * LIVEBOOK_WITHIN_IFRAME - controls if the application is running inside an
+    iframe. Set it to "true" to enable it. If you do enable it, then the application
+    must run with HTTPS.
+
 <!-- Environment variables -->
 
 If running Livebook as a Docker image or an Elixir release, [the environment

config/config.exs

@@ -23,12 +23,14 @@ config :livebook,
   app_service_name: nil,
   app_service_url: nil,
   authentication_mode: :token,
-  learn_notebooks: [],
   feature_flags: [],
+  force_ssl_host: nil,
+  learn_notebooks: [],
   plugs: [],
   shutdown_enabled: false,
-  within_iframe: false,
-  storage: Livebook.Storage.Ets
+  storage: Livebook.Storage.Ets,
+  update_instructions_url: nil,
+  within_iframe: false
 
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.

lib/livebook/config.ex

@@ -143,7 +143,14 @@ defmodule Livebook.Config do
   """
   @spec update_instructions_url() :: String.t() | nil
   def update_instructions_url() do
-    Application.get_env(:livebook, :update_instructions_url)
+    Application.fetch_env!(:livebook, :update_instructions_url)
+  end
+
+  @doc """
+  Returns the force ssl host if any.
+  """
+  def force_ssl_host do
+    Application.fetch_env!(:livebook, :force_ssl_host)
   end
 
   @feature_flags Application.compile_env(:livebook, :feature_flags)

lib/livebook_web/endpoint.ex

@@ -94,11 +94,9 @@ defmodule LivebookWeb.Endpoint do
     end
   end
 
-  @plug_ssl Plug.SSL.init(host: {Application, :get_env, [:livebook, :force_ssl_host, nil]})
+  @plug_ssl Plug.SSL.init(host: {Livebook.Config, :force_ssl_host, []})
   def force_ssl(conn, _opts) do
-    force_ssl_host = Application.get_env(:livebook, :force_ssl_host, nil)
-
-    if force_ssl_host do
+    if Livebook.Config.force_ssl_host() do
       Plug.SSL.call(conn, @plug_ssl)
     else
       conn