What does the runner pipeline in GitLab CI run on?

GitLab Runner is an agent that executes your pipeline jobs. Runners can be run on various platforms:

• Virtual machine or physical server: A traditional method that allows full control over the environment.
• Containers (Docker): Enables running each job in an isolated, standard environment. This is the most popular method.
• Kubernetes: GitLab Runner has an executor for Kubernetes that runs each job in a separate pod.
• Cloud providers: Some cloud providers offer services for running containers or virtual machines that can be used as Runners.
• macOS / Windows: Runners can be run on these operating systems for building specific projects.

When configuring a runner, an "executor" is specified, which determines how the jobs will be executed. The most common are: docker, shell, kubernetes.

Example of defining a runner in the .gitlab-ci.yml file using the docker executor:

stages:
  - build

build_job:
  stage: build
  script:
    - echo "Building project..."

In this example, the build_job will be executed on a runner that has a configured docker executor. GitLab Runner will receive this job and run it in a new Docker container, using the Docker image specified in the runner's configuration or in the .gitlab-ci.yml itself (if image is specified).