diff --git a/.ci/docker-images/Dockerfile b/.ci/docker-images/Dockerfile
index 5fec3c97d26..6e8947c3dcb 100644
--- a/.ci/docker-images/Dockerfile
+++ b/.ci/docker-images/Dockerfile
@@ -24,8 +24,12 @@ RUN apt-get update \
 		aspell-de \
 		hunspell-en-us \
 		git \
+		locales \
  && apt-get clean
 
+# Some tests require en_US.UTF-8 as locale.
+RUN sed -i 's/^# en_US.UTF-8 /en_US.UTF-8 /' /etc/locale.gen && locale-gen
+
 # TODO: Do we need the multiarch-args? What for?
 #RUN debMultiarch="$(dpkg-architecture --query DEB_BUILD_MULTIARCH)" \
 RUN docker-php-ext-configure gd --with-jpeg --with-freetype \
diff --git a/.github/workflows/docker_image.yml b/.github/workflows/docker_image.yml
index a15c7345e5e..3a921c79f7f 100644
--- a/.github/workflows/docker_image.yml
+++ b/.github/workflows/docker_image.yml
@@ -5,6 +5,9 @@ on:
     paths:
       - '.ci/docker-images/*'
       - '.github/workflows/docker_image.yml'
+  schedule:
+    # Rebuild automatically each monday early morning.
+    - cron: "42 5 * * 1"
 
 jobs:
   build_and_push:
diff --git a/tests/Framework/ImapTest.php b/tests/Framework/ImapTest.php
index cd539302f24..cbc2aeb1400 100644
--- a/tests/Framework/ImapTest.php
+++ b/tests/Framework/ImapTest.php
@@ -47,6 +47,11 @@ public function test_convert_criteria()
      */
     public function test_sort_folder_list()
     {
+        // The sorting requires this locale.
+        if (setlocale(\LC_ALL, 'en_US.UTF-8', 'en_US.utf8', 'en_US', 'en-US') === false) {
+            throw new \Error('This test requires `en_US` to be settable as locale, but those appear to not be present in your environment!');
+        }
+
         $_SESSION['imap_delimiter'] = '.';
         $_SESSION['imap_namespace'] = [
             'personal' => null,