Disable Renovate Dockerfile base-image updates in downstream repos

Downstream repos generated from this template were getting Renovate
PRs (e.g. "Update ubuntu Docker tag") for base images that are only
ever meant to be bumped centrally, in template/Dockerfile.jinja.
Add a packageRule to renovate.json.jinja that disables the dockerfile
manager for ubuntu and ghcr.io/diamondlightsource/ubuntu-devcontainer,
mirroring the existing pyenv and github-actions disable rules.

Reported in bluesky/scanspec#211.

Author: coretl

template/renovate.json.jinja

@@ -36,6 +36,17 @@
         "github-actions"
       ],
       "enabled": false
+    },
+    {
+      "description": "Disable Dockerfile base images that are managed by python-copier-template",
+      "matchManagers": [
+        "dockerfile"
+      ],
+      "matchDepNames": [
+        "ubuntu",
+        "ghcr.io/diamondlightsource/ubuntu-devcontainer"
+      ],
+      "enabled": false
     }
   ]
 }