Concourse can run as many VMs as you want. The point is those jobs get pushed to a farm of workers (typically VMs) and then docker is used so you have an epheme

Concourse can run as many VMs as you want. The point is those jobs get pushed to a farm of workers (typically VMs) and then docker is used so you have an ephemeral and repeatable build environment. It is absurdly complex to set up though. I feel there is a gap in the market for a Concourse-like solution but simplified immensely.