chore: Add dockerfile config to dependabot (#98533)

Signed-off-by: Dave Henderson <dave.henderson@grafana.com>

.github/dependabot.yml

@@ -24,3 +24,18 @@ updates:
       time: "02:00"
       timezone: Etc/UTC
     open-pull-requests-limit: 10
+  - package-ecosystem: "docker"
+    directories:
+      - "/"
+      - "/packaging/custom"
+      - "/scripts/verify-repo-update"
+      - "/scripts/build/ci-deploy"
+      - "/scripts/build/ci-windows-test"
+      - "/scripts/build/ci-e2e"
+      - "/scripts/build/ci-wix"
+      - "/scripts/build/ci-msi-build"
+    schedule:
+      interval: "daily"
+      time: "02:00"
+      timezone: Etc/UTC
+    open-pull-requests-limit: 10