More natural api when you are just applying accessory to a single tag

lib/kamal/configuration/accessory.rb

@@ -207,7 +207,9 @@ class Kamal::Configuration::Accessory
     end
 
     def hosts_from_tags
-      if accessory_config.key?("tags")
+      if accessory_config.key?("tag")
+        extract_hosts_from_config_with_tag(accessory_config["tag"])
+      elsif accessory_config.key?("tags")
         accessory_config["tags"].flat_map { |tag| extract_hosts_from_config_with_tag(tag) }
       end
     end

lib/kamal/configuration/docs/accessory.yml

@@ -53,8 +53,10 @@ accessories:
       - mysql-db2
     roles:
       - mysql
+    tag: writer
     tags:
       - writer
+      - reader
 
     # Custom command
     #

lib/kamal/configuration/validator/accessory.rb

@@ -2,8 +2,8 @@ class Kamal::Configuration::Validator::Accessory < Kamal::Configuration::Validat
   def validate!
     super
 
-    if (config.keys & [ "host", "hosts", "roles", "tags" ]).size != 1
-      error "specify one of `host`, `hosts`, `roles` or `tags`"
+    if (config.keys & [ "host", "hosts", "roles", "tag", "tags" ]).size != 1
+      error "specify one of `host`, `hosts`, `roles`, `tag` or `tags`"
     end
 
     validate_docker_options!(config["options"])