Add Docker updates to dependabot config

* Added Docker entries to the dependabot.yml file
* Enables automated updates for Docker dependencies
* Schedule set for daily updates at 06:00 in Europe/Berlin timezone
* Allows up to 99 open pull requests for Docker updates

.github/dependabot.yml

@@ -7,7 +7,6 @@ updates:
     time: "06:00"
     timezone: Europe/Berlin
   open-pull-requests-limit: 99
-
 - package-ecosystem: github-actions
   directory: "/"
   schedule:
@@ -15,3 +14,10 @@ updates:
     time: "06:00"
     timezone: Europe/Berlin
   open-pull-requests-limit: 99
+- package-ecosystem: docker
+  directory: "/"
+  schedule:
+    interval: daily
+    time: "06:00"
+    timezone: Europe/Berlin
+  open-pull-requests-limit: 99