How should I authenticate if I want to use an image from the Gitlab Registry as a base image of another CI build?
According to https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/configuration/advanced-configuration.md#using-a-private-docker-registry I first have to manually login on the runner machine. Somehow it feels strange to login with an existing Gitlab user.
Is there a way to use the CI variable "CI_BUILD_TOKEN" (which is described as "Token used for authenticating with the GitLab Container Registry") for authentication to pull the base image from Gitlab Registry?
EDIT: I found out that I can use images from public projects. But I don't really want to make my docker projects public.
UPDATE: Starting with Gitlab 8.14 you can just use the docker images from the build in docker registry. See https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/configuration/advanced-configuration.md#support-for-gitlab-integrated-registry
Edit 2024: this answer is now eight years old, look at the other answers.
No, this is currently not possible in any elegant way. GitLab should implement explicit credentials for the base images, it will be the most straight-forward and correct solution.
You need to docker login on the GitLab Runner machine. You can't use the gitlab-ci-token since they expire and also project-dependant, so you can't actually use one token for every project. Using your own login is pretty much the only solution available right now (happy to get corrected on this one).