diff --git a/schema.yaml b/schema.yaml
index 47e957b..27901a9 100644
--- a/schema.yaml
+++ b/schema.yaml
@@ -4,6 +4,7 @@ mapping:
     # Global configuration of computing nodes.
     "images":
         type: map
+        required: true
         mapping:
             "default":
                 type: str
@@ -38,6 +39,7 @@ mapping:
     # Resource group definitions.
     "deployment":
         type: map
+        required: true
         mapping:
             "=":
                 type: map
@@ -74,17 +76,21 @@ mapping:
                             - m1.xxlarge
                     "start":
                         type: date
+                        required: false
                         format: "%Y-%m-%d"
                     "end":
                         type: date
+                        required: false
                         format: "%Y-%m-%d"
                     "group":
                         type: str
+                        required: true
                     "image":
                         type: str
                         required: false
                     "volume":
                         type: map
+                        required: false
                         'mapping':
                             "size":
                                 type: int
@@ -99,6 +105,7 @@ mapping:
                                 type: bool
                     "cgroups":
                         type: map
+                        required: false
                         'mapping':
                             "mem_limit_policy":
                                 type: str
@@ -112,3 +119,4 @@ mapping:
                                     min: 1024
                     "docker":
                         type: bool
+                        required: false
diff --git a/synchronize.py b/synchronize.py
index 5af1112..7354742 100644
--- a/synchronize.py
+++ b/synchronize.py
@@ -592,6 +592,18 @@ def template_userdata(
             rendering the template.
     """
     vars_files = frozenset(vars_files)
+
+    environment = Environment(
+        loader=FileSystemLoader(user_data_file.parent),
+        undefined=StrictUndefined,
+    )
+    template = environment.get_template(
+        user_data_file.name,
+    )
+
+    group_defaults = {
+        "docker": False,
+    }
     vars_from_files = (
         reduce(
             lambda x, y: x.update(y),
@@ -601,19 +613,13 @@ def template_userdata(
         else {}
     )
 
-    environment = Environment(
-        loader=FileSystemLoader(user_data_file.parent),
-        undefined=StrictUndefined,
-    )
-    template = environment.get_template(
-        user_data_file.name,
-    )
+    variables = {}
+    for vars_group in (group_defaults, config, group_config, vars_from_files):
+        variables.update(vars_group)
 
     return template.render(
         name=name,
-        **config,
-        **group_config,
-        **vars_from_files,
+        **variables,
     )
 
 
diff --git a/tests.py b/tests.py
index c0ebfef..7992d89 100644
--- a/tests.py
+++ b/tests.py
@@ -1440,6 +1440,7 @@ def test_template_userdata() -> None:
             owner: root:root
             path: /etc/configuration_file
             permissions: "0644"
+            docker: {{ docker }}
     """
     )[1:]
 
@@ -1481,6 +1482,7 @@ def test_template_userdata() -> None:
             owner: root:root
             path: /etc/configuration_file
             permissions: "0644"
+            docker: False
     """
     )[1:-1]
     assert templated == expected