App exec with env file

When calling `kamal app exec` for new non interactive containers, run
the command per role on each server and include the role config
including the environment.

Fixes: https://github.com/basecamp/kamal/issues/492

test/cli/app_test.rb

@@ -159,7 +159,7 @@ class CliAppTest < CliTestCase
 
   test "exec" do
     run_command("exec", "ruby -v").tap do |output|
-      assert_match "docker run --rm dhh/app:latest ruby -v", output
+      assert_match "docker run --rm --env-file .kamal/env/roles/app-web.env dhh/app:latest ruby -v", output
     end
   end