Role of docker-in-docker (dind) service in gitlab ci

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According to the official gitlab documentation, one way to enable docker build within ci pipelines, is to make use of the dind service (in terms of gitlab-ci services).

However, as it is always the case with ci jobs running on docker executors, the docker:latest image is also needed.

Could someone explain:

  • what is the difference between the docker:dind and the docker:latest images?

  • (most importantly): why are both the service and the docker image needed (e.g. as indicated in this example, linked to from the github documentation) to perform e.g. a docker build whithin a ci job? doesn’t the docker:latest image (within which the job will be executed!) incorporate the docker daemon (and I think the docker-compose also), which are the tools necessary for the commands we need (e.g. docker build, docker push etc)?

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For those like me wondering the same question, here is an answer to it :

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