Ensure that the restart policy is unless-stopped

No other restart policy makes sense to don't let it be changed.

Fixes: https://github.com/basecamp/kamal/issues/749

lib/kamal/configuration/validator.rb

@@ -168,4 +168,10 @@ class Kamal::Configuration::Validator
       unknown_keys.reject! { |key| extension?(key) } if allow_extensions?
       unknown_keys_error unknown_keys if unknown_keys.present?
     end
+
+    def validate_docker_options!(options)
+      if options
+        error "Cannot set restart policy in docker options, unless-stopped is required" if options["restart"]
+      end
+    end
 end

lib/kamal/configuration/validator/accessory.rb

@@ -5,5 +5,7 @@ class Kamal::Configuration::Validator::Accessory < Kamal::Configuration::Validat
     if (config.keys & [ "host", "hosts", "roles" ]).size != 1
       error "specify one of `host`, `hosts` or `roles`"
     end
+
+    validate_docker_options!(config["options"])
   end
 end

lib/kamal/configuration/validator/role.rb

@@ -6,6 +6,7 @@ class Kamal::Configuration::Validator::Role < Kamal::Configuration::Validator
       validate_servers!(config)
     else
       super
+      validate_docker_options!(config["options"])
     end
   end
 end