Seed docker mirrors by pulling once per mirror first

Find the first registry mirror on each host. If we find any, pull the
images on one host per mirror, then do the remainder concurrently.

The initial pulls will seed the mirrors ensuring that we pull the image
from Docker Hub once each.

This works best if there is only one mirror on each host.

test/commands/builder_test.rb

@@ -200,6 +200,11 @@ class CommandsBuilderTest < ActiveSupport::TestCase
     assert_equal [ "unix:///var/run/docker.sock", "ssh://host" ], command.config_context_hosts
   end
 
+  test "mirror count" do
+    command = new_builder_command
+    assert_equal "docker info --format '{{index .RegistryConfig.Mirrors 0}}'", command.first_mirror.join(" ")
+  end
+
   private
     def new_builder_command(additional_config = {})
       Kamal::Commands::Builder.new(Kamal::Configuration.new(@config.merge(additional_config), version: "123"))