Dependabot: add docker dependencies

Summary:
/shrug

Signed-off-by: Yongmin Hong <revi@omglol.email>

Test Plan: Wait for Monday for dependabot to trigger.

Reviewers: O1 revi & automations, revi

Reviewed By: O1 revi & automations, revi

Differential Revision: https://issuetracker.revi.xyz/D135

.github/dependabot.yml

@@ -4,12 +4,26 @@
 version: 2
 updates:
 
+  # Maintain dependencies for Docker
+  - package-ecosystem: "docker"
+    assignees:
+      - "revi"
+    # Dockerfiles are stored in `container/lighthouse-ci-server/.
+    directory: "container/lighthouse-ci-server"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+      time: "10:00"
+      timezone: "Asia/Seoul"
+    target-branch: "master"
+    labels: []
   # Maintain dependencies for GitHub Actions
   - package-ecosystem: "github-actions"
     assignees:
       - "revi"
     # Workflow files stored in the default location of `.github/workflows`.
-    # (You don't need to specify `/.github/workflows` for `directory`. You can use `directory: "/"`.)
+    # You don't need to specify `/.github/workflows` for `directory`.
+    # You can use `directory: "/"`.
     directory: "/"
     schedule:
       interval: "weekly"