Ignore renovate updates for ubuntu and ubuntu-devcontainer

[coretl]

Something like:

{
  "packageRules": [
    {
      "matchManagers": ["dockerfile"],
      "matchDatasources": ["docker"],
      "matchPackageNames": ["ubuntu", "ubuntu-devcontainer"],
      "matchUpdateTypes": ["digest"],
      "enabled": false
    }
  ]
}