ci: add dependabot configuration

This commit adds dependabot configuration to get updates for
Dockerfiles and GitHub Actions.

.github/dependabot.yaml

@@ -0,0 +1,21 @@
+version: 2
+updates:
+    - package-ecosystem: "docker"
+      directory: "/"
+      schedule:
+          interval: "weekly"
+      labels:
+          - "dependencies"
+      commit-message:
+          prefix: "build"
+          include: "scope"
+
+    - package-ecosystem: "github-actions"
+      directory: "/"
+      schedule:
+          interval: "weekly"
+      labels:
+          - "dependencies"
+      commit-message:
+          prefix: "build"
+          include: "scope"