Add a singular role

lib/kamal/configuration/accessory.rb

@@ -201,7 +201,9 @@ class Kamal::Configuration::Accessory
     end
 
     def hosts_from_roles
-      if accessory_config.key?("roles")
+      if accessory_config.key?("role")
+       config.role(accessory_config["role"])&.hosts
+      elsif accessory_config.key?("roles")
         accessory_config["roles"].flat_map { |role| config.role(role)&.hosts }
       end
     end
@@ -231,6 +233,8 @@ class Kamal::Configuration::Accessory
     def ensure_valid_roles
       if accessory_config["roles"] && (missing_roles = accessory_config["roles"] - config.roles.map(&:name)).any?
         raise Kamal::ConfigurationError, "accessories/#{name}: unknown roles #{missing_roles.join(", ")}"
+      elsif accessory_config["role"] && !config.role(accessory_config["role"])
+        raise Kamal::ConfigurationError, "accessories/#{name}: unknown role #{accessory_config["role"]}"
       end
     end
 end

lib/kamal/configuration/docs/accessory.yml

@@ -46,11 +46,12 @@ accessories:
 
     # Accessory hosts
     #
-    # Specify one of `host`, `hosts`, `roles`, or `tags`:
+    # Specify one of `host`, `hosts`, `role`, `roles`, `tag` or `tags`:
     host: mysql-db1
     hosts:
       - mysql-db1
       - mysql-db2
+    role: mysql
     roles:
       - mysql
     tag: writer

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", "tag", "tags" ]).size != 1
-      error "specify one of `host`, `hosts`, `roles`, `tag` or `tags`"
+    if (config.keys & [ "host", "hosts", "role", "roles", "tag", "tags" ]).size != 1
+      error "specify one of `host`, `hosts`, `role`, `roles`, `tag` or `tags`"
     end
 
     validate_docker_options!(config["options"])