[Renovate] Auto update common ci-agent-images

[breskeby]

This should help us keeping buildkite agent images up-to-date in our pipelines.

This is a follow up on https://github.com/elastic/elasticsearch/pull/134118/files#r2321516783

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[jamietanna]

Hey @breskeby this isn't necessarily going to work as is.

The preset we're including in this PR will add better information for the docker.elastic.co/ci-agent-images/eck-region/buildkite-agent image that we're using , but it won't introduce updates for the image, as we're using :only-chainguard

Gimme a sec, and I'll share a diff that should do it

[jamietanna]

The full renovate.json I'd recommend is:

{
  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
  "extends": [
    "github>elastic/renovate-config:only-chainguard",
    "github>elastic/renovate-config:ci-agent-images",
    ":disableDependencyDashboard"
  ],
  "schedule": [
    "after 1pm on tuesday"
  ],
  "labels": [
    ">non-issue",
    ":Delivery/Packaging",
    ":Delivery/Build",
    "Team:Delivery",
    "auto-merge-without-approval"
  ],
  "baseBranches": [
    "main",
    "9.1",
    "9.0",
    "8.19",
    "8.18",
    "7.17"
  ],
  "packageRules": [
    {
      "description": "Explicitly enable ci-agent-image updates",
      "matchPackageNames": [
        "/docker.elastic.co/ci-agent-images/.*/"
      ],
      "enabled": true
    }
  ]
}

Note the explicit enabling of docker.elastic.co/ci-agent-images/ images