There are a few uses cases emerging to be able to run arbitrary Docker images on CI. Usually they come as a request to be able to use docker-compose
The first use case was to run integration tests for Fresh. Since we can not run arbitrary Docker images on the CI WMCS hosts, we went using Qemu and some Debian snapshot (T250808). That is merely a homegrown solution for what was fulfilled by Nodepool a few years ago.
It is a single WMCS instance for now and serving a single project, this task is to track over use cases. Probably we should revisit the decision that has been made on T250808 and find a better long term solution (maybe helm chart + K8s, some VM orchestration system).