Docker Plugin: How do container agents get provisioned?

[ma...@jochen-fuerbacher.de]

Hello,

I was wondering, how new container agents get provisioned by the Docker Plugin.

Scenario:
- I have 3 Jobs. Each of them do nothing but "sleep 500" (so that those Jobs run for a longer time).
- Count of executors on master node is set to 0.
- 1 connected (non-Docker) Agent with 1 executor.
- Connected Docker engine with Container cap of 30.

When the non-Docker Agent is offline and I start all three Jobs, Jenkins creates three Agents running in Docker containers.
But when the non-Docker Agent is online, 1 Job gets built on the non-Docker Agent, but the other two Jobs remain in the build queue until the first build job is finished. However sometimes, when 1 job gets built on the non-Docker Agent, Jenkins creates two (or more) Agents running in Docker containers and the remaining Jobs get built on those Agents now.

So my question is: How do new Docker container Agents get provisioned and when?

(Docker Plugin version 0.16.1, Jenkins core version: 2.7.2)

Thank you.