chore(github): expand dependabot to GitHub Actions + Docker

Existing npm group covered runtime deps. Adds weekly bumps for the
.github/workflows/* action versions and the base image referenced in
Dockerfile so neither falls behind silently.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/dependabot.yml

@@ -10,3 +10,13 @@ updates:
         applies-to: security-updates
         update-types:
           - "patch"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 3
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 3