Question about running Travis CI Build locally

Hi,

I was trying to run some the Travis CI Build locally by using docker. I noticed that some worker instance cannot be found on the Dockerhub (https://hub.docker.com/r/travisci/ci-garnet)

Could you info me where to find the docker base images which are not on the Dockerhub?

Here here some examples:
In this job, under worker information, the instance “travis-ci-garnet-trusty-1512502259-986baf0” cannot found anywhere.

However,
In this job, under worker information, the instance “travisci/ci-garnet:packer-1512502276-986baf0” can be found.

Thanks!