How to publish docker images to docker hub from gitlab-ci - gitlab-ci

Question

Gitlab provides a .gitlab-ci.yml template for building and publishing images to its own registry (click "new file" in one of your project, select .gitlab-ci.yml and docker). The file looks like this and it works out of the box :)
# This file is a template, and might need editing before it works on your project.
# Official docker image.
image: docker:latest
services:
- docker:dind
before_script:
- docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
build-master:
stage: build
script:
- docker build --pull -t "$CI_REGISTRY_IMAGE" .
- docker push "$CI_REGISTRY_IMAGE"
only:
- master
build:
stage: build
script:
- docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG" .
- docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG"
except:
- master
But by default, this will publish to gitlab's registry. How can we publish to docker hub instead?

Answer 1

No need to change that .gitlab-ci.yml at all, we only need to add/replace the environment variables in project's pipeline settings.
1. Find the desired registry url
Using hub.docker.com won't work, you'll get the following error:
Error response from daemon: login attempt to https://hub.docker.com/v2/ failed with status: 404 Not Found
Default docker hub registry url can be found like this:
docker info | grep Registry
Registry: https://index.docker.io/v1/
index.docker.io is what I was looking for.
2. Set the environment variables in gitlab settings
I wanted to publish gableroux/unity3d images using gitlab-ci, here's what I used in Gitlab's project > Settings > CI/CD > Variables
CI_REGISTRY_USER=gableroux
CI_REGISTRY_PASSWORD=********
CI_REGISTRY=docker.io
CI_REGISTRY_IMAGE=index.docker.io/gableroux/unity3d
CI_REGISTRY_IMAGE is important to set.
It defaults to registry.gitlab.com/<username>/<project>
regsitry url needs to be updated so use index.docker.io/<username>/<project>
Since docker hub is the default registry when using docker, you can also use <username>/<project> instead. I personally prefer when it's verbose so I kept the full registry url.
This answer should also cover other registries, just update environment variables accordingly. 🙌

Answer 2

To expand on the GabLeRoux's answer,
I had issues on the pushing stage of the GitLab CI build:
denied: requested access to the resource is denied
By changing my CI_REGISTRY to docker.io (remove the index.) I was able to successfully push.