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/cli/build_test.rb

@@ -169,12 +169,41 @@ class CliBuildTest < CliTestCase
 
   test "pull" do
     run_command("pull").tap do |output|
+      assert_match /docker info --format '{{index .RegistryConfig.Mirrors 0}}'/, output
       assert_match /docker image rm --force dhh\/app:999/, output
       assert_match /docker pull dhh\/app:999/, output
       assert_match "docker inspect -f '{{ .Config.Labels.service }}' dhh/app:999 | grep -x app || (echo \"Image dhh/app:999 is missing the 'service' label\" && exit 1)", output
     end
   end
 
+  test "pull with mirror" do
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :info, "--format '{{index .RegistryConfig.Mirrors 0}}'")
+      .returns("registry-mirror.example.com")
+      .at_least_once
+
+    run_command("pull").tap do |output|
+      assert_match /Pulling image on 1\.1\.1\.\d to seed the mirror\.\.\./, output
+      assert_match "Pulling image on remaining hosts...", output
+      assert_match /docker pull dhh\/app:999/, output
+      assert_match "docker inspect -f '{{ .Config.Labels.service }}' dhh/app:999 | grep -x app || (echo \"Image dhh/app:999 is missing the 'service' label\" && exit 1)", output
+    end
+  end
+
+  test "pull with mirrors" do
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :info, "--format '{{index .RegistryConfig.Mirrors 0}}'")
+      .returns("registry-mirror.example.com", "registry-mirror2.example.com")
+      .at_least_once
+
+    run_command("pull").tap do |output|
+      assert_match /Pulling image on 1\.1\.1\.\d, 1\.1\.1\.\d to seed the mirrors\.\.\./, output
+      assert_match "Pulling image on remaining hosts...", output
+      assert_match /docker pull dhh\/app:999/, output
+      assert_match "docker inspect -f '{{ .Config.Labels.service }}' dhh/app:999 | grep -x app || (echo \"Image dhh/app:999 is missing the 'service' label\" && exit 1)", output
+    end
+  end
+
   test "create" do
     run_command("create").tap do |output|
       assert_match /docker buildx create --use --name kamal-app-multiarch/, output

test/cli/main_test.rb

@@ -122,6 +122,11 @@ class CliMainTest < CliTestCase
       .with(:docker, :buildx, :inspect, "kamal-app-multiarch", "> /dev/null")
       .returns("")
 
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :info, "--format '{{index .RegistryConfig.Mirrors 0}}'")
+      .returns("")
+      .at_least_once
+
     assert_raises(Kamal::Cli::LockError) do
       run_command("deploy")
     end
@@ -155,6 +160,11 @@ class CliMainTest < CliTestCase
       .with(:docker, :buildx, :inspect, "kamal-app-multiarch", "> /dev/null")
       .returns("")
 
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :info, "--format '{{index .RegistryConfig.Mirrors 0}}'")
+      .returns("")
+      .at_least_once
+
     assert_raises(SSHKit::Runner::ExecuteError) do
       run_command("deploy")
     end

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"))