Support rootless Docker in Docker

Description

I'm concerned about my own use of privileged: true, and want to mitigate it. Is the new support for rootless DinD something that can be used? This could then be documented as an approach! I'd like to see some documentation about it alongside DinD and Kaniko.

Proposal

We explore this option together, and document it as an option to using docker:dind with priviledged: true or Kaniko here.

Links to related issues and merge requests / references

• About rootless docker:dind: https://www.youtube.com/watch?v=Qq78zfXUq18
• Docker release notes: https://docs.docker.com/engine/release-notes/#19030
• This repos current documentation about DinD and Kaniko: https://docs.gitlab.com/runner/executors/kubernetes.html#using-dockerdind