Use service if image is blank and registry is local

lib/kamal/configuration.rb

@@ -6,7 +6,7 @@
 require "net/ssh/proxy/jump"
 
 class Kamal::Configuration
-  delegate :service, :image, :labels, :hooks_path, to: :raw_config, allow_nil: true
+  delegate :service, :labels, :hooks_path, to: :raw_config, allow_nil: true
   delegate :argumentize, :optionize, to: Kamal::Utils
 
   attr_reader :destination, :raw_config, :secrets
@@ -149,6 +149,13 @@ def proxy_hosts
     proxy_roles.flat_map(&:hosts).uniq
   end
 
+  def image
+    name = raw_config&.image.presence
+    name ||= raw_config&.service if registry.local?
+
+    name
+  end
+
   def repository
     [ registry.server, image ].compact.join("/")
   end
@@ -306,10 +313,12 @@ def ensure_destination_if_required
     end
 
     def ensure_required_keys_present
-      %i[ service image registry servers ].each do |key|
+      %i[ service registry servers ].each do |key|
         raise Kamal::ConfigurationError, "Missing required configuration for #{key}" unless raw_config[key].present?
       end
 
+    raise Kamal::ConfigurationError, "Missing required configuration for image" if image.blank?
+
       unless role(primary_role_name).present?
         raise Kamal::ConfigurationError, "The primary_role #{primary_role_name} isn't defined"
       end

test/configuration_test.rb

@@ -43,6 +43,19 @@ class ConfigurationTest < ActiveSupport::TestCase
     end
   end
 
+  test "image uses service name if registry is local" do
+    assert_equal "app", Kamal::Configuration.new(@deploy.tap {
+      _1[:registry] = { "server" => "localhost:5000" }
+      _1.delete(:image)
+    }).image
+  end
+
+  test "image uses image if registry is local" do
+    assert_equal "dhh/app", Kamal::Configuration.new(@deploy.tap {
+      _1[:registry] = { "server" => "localhost:5000" }
+    }).image
+  end
+
   test "service name valid" do
     assert_nothing_raised do
       Kamal::Configuration.new(@deploy.tap { _1[:service] = "hey-app1_primary" })