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/commands/registry_test.rb

@@ -55,6 +55,14 @@ class CommandsRegistryTest < ActiveSupport::TestCase
       registry.logout.join(" ")
   end
 
+  test "registry setup" do
+    assert_equal "docker start kamal-docker-registry || docker run --detach -p :5000 --name kamal-docker-registry registry:2", registry.setup.join(" ")
+  end
+
+  test "registry remove" do
+    assert_equal "docker stop kamal-docker-registry && docker rm kamal-docker-registry", registry.remove.join(" ")
+  end
+
   private
     def registry
       Kamal::Commands::Registry.new Kamal::Configuration.new(@config)