[dependabot] Manage Docker versions

Author: IsaacG

.github/dependabot.yml

@@ -3,7 +3,13 @@ version: 2
 updates:
 
   # Keep dependencies for GitHub Actions up-to-date
-  - package-ecosystem: 'github-actions'
-    directory: '/'
+  - package-ecosystem: github-actions
+    directory: /
     schedule:
-      interval: 'monthly'
+      interval: monthly
+
+  # Enable updates to the dependencies of our Dockerfile
+  - package-ecosystem: docker
+    directory: /
+    schedule:
+      interval: monthly