ci: track pip, docker and action updates weekly with a cooldown

.github/dependabot.yml

@@ -0,0 +1,36 @@
+version: 2
+updates:
+  - package-ecosystem: pip
+    directory: /
+    schedule:
+      interval: weekly
+    # Never install a release younger than a week: gives the ecosystem time
+    # to detect and yank compromised versions before we ever resolve them.
+    cooldown:
+      default-days: 7
+    # One combined PR per week instead of one per package.
+    groups:
+      minor-and-patch:
+        applies-to: version-updates
+        update-types:
+          - minor
+          - patch
+    open-pull-requests-limit: 5
+
+  - package-ecosystem: docker
+    directory: /
+    schedule:
+      interval: weekly
+    cooldown:
+      default-days: 7
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    # One combined PR; action majors are routine tag moves and CI
+    # validates the grouped PR anyway.
+    groups:
+      actions:
+        patterns:
+          - "*"