Per destination latest image tag

We tag the deployed image with the latest image tag when deploying. But
if we are deploying multiple destinations to the same host they'll
conflict with each other.

lib/kamal/configuration.rb

@@ -128,7 +128,7 @@ class Kamal::Configuration
   end
 
   def latest_image
-    "#{repository}:latest"
+    "#{repository}:#{[ "latest", *destination ].join("-")}"
   end
 
   def service_with_version

test/configuration_test.rb

@@ -130,6 +130,16 @@ class ConfigurationTest < ActiveSupport::TestCase
     assert_equal "ghcr.io/dhh/app:missing", config.absolute_image
   end
 
+  test "latest image" do
+    assert_equal "dhh/app:latest", @config.latest_image
+  end
+
+  test "latest image with destination" do
+    dest_config_file = Pathname.new(File.expand_path("fixtures/deploy_for_dest.yml", __dir__))
+    config = Kamal::Configuration.create_from config_file: dest_config_file, destination: "world"
+    assert_equal "registry.digitalocean.com/dhh/app:latest-world", config.latest_image
+  end
+
   test "service with version" do
     assert_equal "app-missing", @config.service_with_version
   end