build: configure dependabot to update packaging Dockerfiles

See dependabot/dependabot-core#2824

Followup on #2306

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

.github/dependabot.yml

@@ -1,19 +1,24 @@
 # To get started with Dependabot version updates, you'll need to specify which
 # package ecosystems to update and where the package manifests are located.
 # Please see the documentation for all configuration options:
-# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
-
+# * https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+# * https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
 version: 2
 updates:
   - package-ecosystem: "gomod"
-    directory: "/" # Location of package manifests
+    directory: "/"
     schedule:
       interval: "weekly"
   - package-ecosystem: "github-actions"
-    directory: "/" # Location of package manifests https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#directory
+    directory: "/" # For GitHub Actions, set the directory to / to check for workflow files in .github/workflows
     schedule:
       interval: "weekly"
-  - package-ecosystem: "docker" # Keep Docker dependencies up to date
+  # Keep Docker dependencies up to date
+  - package-ecosystem: "docker"
     directory: "/"
     schedule:
       interval: "daily"
+  - package-ecosystem: "docker"
+    directory: "/packaging"
+    schedule:
+      interval: "daily"