diff --git a/tests/examples/dsl_configurator.py b/tests/examples/dsl_configurator.py
index 43b61756..47b1c45d 100644
--- a/tests/examples/dsl_configurator.py
+++ b/tests/examples/dsl_configurator.py
@@ -35,7 +35,7 @@ def run(self, task: TaskView) -> bool:
         extra = self.a_requirement.extra  # type: ignore
         self.a_requirement.copy_of_extra = extra  # type: ignore
 
-        self.data_list.append(MyDataType())
+        self.data_list.append(MyDataType().extend(additional=1))
         self.data_list[0].ports.source = port80
         self.data_list[0].ports.target = tosca.datatypes.NetworkPortDef(8080)
         return True
diff --git a/tests/test_constraints.py b/tests/test_constraints.py
index a6381ee3..63ba8d38 100644
--- a/tests/test_constraints.py
+++ b/tests/test_constraints.py
@@ -301,6 +301,7 @@ def test_computed_properties():
                 "source": 80,
                 "source_range": None,
             },
+
             "a_list": [1],
             "data_list": [
                 {
@@ -310,7 +311,8 @@ def test_computed_properties():
                         "target_range": None,
                         "source": 80,
                         "source_range": None,
-                    }
+                    },
+                    "additional": 1
                 }
             ],
             "extra": "extra",
diff --git a/tests/test_dsl.py b/tests/test_dsl.py
index 3d40e389..660ef987 100644
--- a/tests/test_dsl.py
+++ b/tests/test_dsl.py
@@ -782,6 +782,7 @@ def test_envvar_type():
     import tosca
     from tosca import Property, DEFAULT
     import unfurl
+    from unfurl.configurators.templates.docker import unfurl_datatypes_DockerContainer
 
     with tosca.set_evaluation_mode("spec"):
 
@@ -820,8 +821,10 @@ class pcls(tosca.InstanceProxy):
         generic_envvars = unfurl.datatypes.EnvironmentVariables(DBASE="aaaa", URL=True)
 
         assert generic_envvars.to_yaml() == {"DBASE": "aaaa", "URL": True}
-        assert OpenDataType(a=1, b="b").to_yaml() == {"a": 1, "b": "b"}
+        assert OpenDataType(a=1, b="b").extend(c="c").to_yaml() == {"a": 1, "b": "b", "c": "c"}
         assert Namespace.MyDataType(name="foo").to_yaml() == {"name": "foo"}
+        # make sure DockerContainer is an OpenDataType
+        unfurl_datatypes_DockerContainer().extend(labels=dict(foo="bar"))
 
 
 test_relationship_yaml = """
diff --git a/tosca-package/tosca/_tosca.py b/tosca-package/tosca/_tosca.py
index ccc7f827..395cb296 100644
--- a/tosca-package/tosca/_tosca.py
+++ b/tosca-package/tosca/_tosca.py
@@ -1925,6 +1925,7 @@ def _make_dataclass(cls):
     # we need _Tosca_Fields not dataclasses.Fields
     # so for any declarations of tosca fields (properties, requirements, etc)
     # missing a _Tosca_Fields, set one before calling _process_class()
+    global_state.mode = "spec"
     global_state._in_process_class = True
     try:
         annotations = cls.__dict__.get("__annotations__")
@@ -3183,6 +3184,11 @@ def __init__(self, _name="", **kw):
                 self.__dict__[k] = kw.pop(k)
         super().__init__(_name, **kw)
 
+    def extend(self, **kw) -> Self:
+        "Add undeclared properties to the data type."
+        self.__dict__.update(kw)
+        return self
+
 
 class CapabilityType(_OwnedToscaType):
     _type_section: ClassVar[str] = "capability_types"
diff --git a/unfurl/configurators/templates/docker.py b/unfurl/configurators/templates/docker.py
index 7ec55ab2..92593613 100644
--- a/unfurl/configurators/templates/docker.py
+++ b/unfurl/configurators/templates/docker.py
@@ -22,15 +22,15 @@
     ToscaOutputs,
     operation,
     valid_values,
+    OpenDataType
 )
 import tosca
 import unfurl.configurators.ansible
 from unfurl.tosca_plugins.artifacts import *
 
 
-class unfurl_datatypes_DockerContainer(DataType):
+class unfurl_datatypes_DockerContainer(OpenDataType):
     _type_name = "unfurl.datatypes.DockerContainer"
-    _type_metadata = {"additionalProperties": True}
     environment: Union["unfurl.datatypes.EnvironmentVariables", None] = Property(
         factory=lambda: (unfurl.datatypes.EnvironmentVariables())
     )