Add Docker and GitHub Actions to Dependabot config (#96)

The existing Dependabot configuration only tracked Go module updates,
leaving Docker base images and GitHub Actions workflow files without
automated update notifications.

Add weekly update schedules for both ecosystems so that the ceph/ceph
container base image and shared CI actions stay up to date.

Author: teran

.github/dependabot.yml

@@ -5,7 +5,17 @@
 
 version: 2
 updates:
-  - package-ecosystem: "gomod" # See documentation for possible values
-    directory: "/" # Location of package manifests
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
     schedule:
       interval: "weekly"