Set up renovate-bot to update merge-train Docker base image version automatically

Automate Docker base image updates in the merge-train project (https://gitlab.com/gitlab-org/merge-train) by setting up renovate-bot. This will ensure the base image stays up to date with security and stability fixes while avoiding unexpected breaking changes from manual or infrequent updates.

This ticket was created from INC-4071 and was automatically exported by incident.io 🔥