diff --git a/container/tools/config/helper_dict.py b/container/tools/config/helper_dict.py
new file mode 100644
index 0000000..c590dc0
--- /dev/null
+++ b/container/tools/config/helper_dict.py
@@ -0,0 +1,36 @@
+# Copyright (C) 2019 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import copy
+
+
+def left_outer_join(dict_a, dict_b):
+    merged_dict = copy.deepcopy(dict_a)
+    merged_dict = _recursive_left_outer_join(merged_dict, dict_b)
+
+    return merged_dict
+
+
+def _recursive_left_outer_join(dict_a, dict_b):
+    for key, value in dict_a.items():
+        if key not in dict_b:
+            continue
+        if isinstance(value, dict):
+            if not isinstance(dict_b[key], dict):
+                raise ValueError("Expected dictionary as value of key '%s'" % key)
+            dict_a[key] = _recursive_left_outer_join(value, dict_b[key])
+        else:
+            dict_a[key] = dict_b[key]
+
+    return dict_a
diff --git a/container/tools/config/parser.py b/container/tools/config/parser.py
index f370d23..b55bf08 100644
--- a/container/tools/config/parser.py
+++ b/container/tools/config/parser.py
@@ -16,6 +16,8 @@
 
 import yaml
 
+from .helper_dict import left_outer_join
+
 DEFAULTS = {
     "gerrit": {"url": None, "user": "admin", "password": "secret"},
     "testrun": {"duration": None},
@@ -47,9 +49,8 @@
 
     def parse(self):
         if self.args["config_file"]:
-            for category, category_dict in self._parse_config_file().items():
-                for option, value in category_dict.items():
-                    self.config[category][option] = value
+            config_from_file = self._parse_config_file()
+            self.config = left_outer_join(DEFAULTS, config_from_file)
 
         self._apply_args()