Accessories aren't required to publish a port

Allows for background accessories like schedulers that don't act
as typical network service dependencies and have no port to expose.

lib/mrsk/configuration/accessory.rb

@@ -20,13 +20,15 @@ class Mrsk::Configuration::Accessory
   end
 
   def port
-    if specifics["port"].to_s.include?(":")
-      specifics["port"]
-    else
-      "#{specifics["port"]}:#{specifics["port"]}"
+    if port = specifics["port"]&.to_s
+      port.include?(":") ? port : "#{port}:#{port}"
     end
   end
 
+  def publish_args
+    argumentize "--publish", port if port
+  end
+
   def labels
     default_labels.merge(specifics["labels"] || {})
   end