feat(buildkite): automagically detect Docker images in Buildkite configuration files

[jamietanna]

Currently, we support the extraction of Buildkite plugins (https://docs.renovatebot.com/modules/manager/buildkite/), but do not pick up the image: definition for Docker images

For instance:

# https://github.com/elastic/elasticsearch/blob/d5a05ecca74cffa0d2d036962284d0a661fbf340/.buildkite/pipelines/java-ea-check-new-build.yml#L6
    agents:
      image: "docker.elastic.co/ci-agent-images/eck-region/buildkite-agent:1.15"

Note that agent images don't have to be Docker:

# NOTE that this isn't a Docker image
# https://github.com/elastic/kibana/blob/b1de364cf0128e75d7befac2d8f0031d31e27cd4/.buildkite/pipelines/fips.yml#L7
agents:
  provider: 'gcp'
  image: 'family/kibana-fips-ubuntu-2404'

Via, it's possible to craft a Regex to do this, but we should add first-class support.