diff --git a/resources/build.defaults.yaml b/resources/build.defaults.yaml
index ffe7ae3..c5c73a5 100644
--- a/resources/build.defaults.yaml
+++ b/resources/build.defaults.yaml
@@ -1,5 +1,8 @@
+---
 drupal:
   docroot: web
+  simpletest:
+    db: sqlite://localhost//dev/shm/test.sqlite
 
 experimental:
   runStaticAnalysisOnTests: false
diff --git a/src/Command/CreateFinalConfigurationDataCommand.php b/src/Command/CreateFinalConfigurationDataCommand.php
index 264da31..fee7ad1 100644
--- a/src/Command/CreateFinalConfigurationDataCommand.php
+++ b/src/Command/CreateFinalConfigurationDataCommand.php
@@ -31,7 +31,7 @@ final class CreateFinalConfigurationDataCommand
 
         // Flatten the new `parameters` into the main configuration.
         if (isset($configurationData['parameters'])) {
-            $configurationData = array_merge($configurationData, [...$configurationData['parameters']]);
+            $configurationData = array_replace_recursive($configurationData, [...$configurationData['parameters']]);
         }
 
         // `flake` renamed to `nix`.
diff --git a/src/DataTransferObject/ConfigDto.php b/src/DataTransferObject/ConfigDto.php
index 293be12..d9b1b46 100644
--- a/src/DataTransferObject/ConfigDto.php
+++ b/src/DataTransferObject/ConfigDto.php
@@ -105,7 +105,15 @@ final class ConfigDto
 
     #[Assert\Collection(
         allowExtraFields: false,
-        fields: ['docroot' => new Assert\Choice([null, 'web', 'docroot'])],
+        fields: [
+            'docroot' => new Assert\Choice([null, 'web', 'docroot']),
+
+            'simpletest' => new Assert\Optional(
+                new Assert\Collection([
+                    'db' => new Assert\Optional(new Assert\Type('string')),
+                ]),
+            ),
+        ],
     )]
     public array $drupal;
 
diff --git a/templates/php/drupal/phpunit.xml.dist.twig b/templates/php/drupal/phpunit.xml.dist.twig
index 155926d..ae59632 100644
--- a/templates/php/drupal/phpunit.xml.dist.twig
+++ b/templates/php/drupal/phpunit.xml.dist.twig
@@ -17,7 +17,7 @@
     <env name="MINK_DRIVER_ARGS_WEBDRIVER" value=''/>
     <env name="MINK_DRIVER_CLASS" value=''/>
     <env name="SIMPLETEST_BASE_URL" value="http://web"/>
-    <env name="SIMPLETEST_DB" value="sqlite://localhost//dev/shm/test.sqlite"/>
+    <env name="SIMPLETEST_DB" value="{{ drupal.simpletest.db }}"/>
 
     <ini name="error_reporting" value="32767"/>
     <ini name="memory_limit" value="-1"/>