Added docker to dependabot.yml config (#2316)

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

.github/dependabot.yml

@@ -5,8 +5,19 @@ updates:
     schedule:
       interval: "weekly"
     reviewers:
-      - "georglauterbach"
-      - "wernerfred"
+      - "docker-mailserver/maintainers"
     labels:
       - "area/ci"
+      - "kind/update"
       - "priority/low"
+
+  - package-ecosystem: "docker"
+    directory: /
+    schedule:
+      interval: "weekly"
+    reviewers:
+      - "docker-mailserver/maintainers"
+    labels:
+      - "area/ci"
+      - "kind/update"
+      - "priority/medium"