Use local registry for app images

Allow applications to be deployed without needing to set up a repository
in a remote Docker registry.

If the registry server starts with `localhost`, Kamal will start a local
docker registry on that port and push the app image to it.

Then when pulling the image onto the servers, we use net-ssh to forward
the that port from the app server to the deployment server.

This will allow the deployment server to pull the image from the
registry as if it were local, meaning we don't need to set up a cert.

test/integration/main_test.rb

@@ -29,7 +29,7 @@ class MainTest < IntegrationTest
     assert_match /App Host: vm1/, details
     assert_match /App Host: vm2/, details
     assert_match /basecamp\/kamal-proxy:#{Kamal::Configuration::PROXY_MINIMUM_VERSION}/, details
-    assert_match /registry:4443\/app:#{first_version}/, details
+    assert_match /localhost:5000\/app:#{first_version}/, details
 
     audit = kamal :audit, capture: true
     assert_match /Booted app version #{first_version}.*Booted app version #{second_version}.*Booted app version #{first_version}.*/m, audit
@@ -63,8 +63,8 @@ class MainTest < IntegrationTest
     assert_equal [ "vm1", "vm2" ], config[:hosts]
     assert_equal "vm1", config[:primary_host]
     assert_equal version, config[:version]
-    assert_equal "registry:4443/app", config[:repository]
-    assert_equal "registry:4443/app:#{version}", config[:absolute_image]
+    assert_equal "localhost:5000/app", config[:repository]
+    assert_equal "localhost:5000/app:#{version}", config[:absolute_image]
     assert_equal "app-#{version}", config[:service_with_version]
     assert_equal [], config[:volume_args]
     assert_equal({ user: "root", port: 22, keepalive: true, keepalive_interval: 30, log_level: :fatal }, config[:ssh_options])