Add hyphen to the allowed character list for service name

lib/kamal/configuration.rb

@@ -284,7 +284,7 @@ class Kamal::Configuration
     end
 
     def ensure_valid_service_name
-      raise ArgumentError, "Service name can only include alphanumeric characters and hyphens" unless raw_config[:service] =~ /^[a-z0-9-]+$/
+      raise ArgumentError, "Service name can only include alphanumeric characters, hyphens, and underscores" unless raw_config[:service] =~ /^[a-z0-9-_]+$/
 
       true
     end

test/configuration_test.rb

@@ -43,7 +43,7 @@ class ConfigurationTest < ActiveSupport::TestCase
   end
 
   test "service name valid" do
-    assert Kamal::Configuration.new(@deploy.tap { _1[:service] = "hey-app1" }).valid?
+    assert Kamal::Configuration.new(@deploy.tap { _1[:service] = "hey-app1_primary" }).valid?
   end
 
   test "service name invalid" do