Gitlab-runner, kubernetes, concurrency > 1

Docker Engine General
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5

I think the answer is in the image description on Docker Hub

http://hub.docker.com/_/docker

TLS

Starting in 18.09+, the dind variants of this image will automatically generate TLS certificates in the directory specified by the DOCKER_TLS_CERTDIR environment variable.
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To disable this image behavior, simply override the container command or entrypoint to run dockerd directly (... docker:dind dockerd ... or ... --entrypoint dockerd docker:dind ... ).