Can you list the types of runners used in GitLab CI?

The following types of runners are used in GitLab CI:

• Shared Runners — common runners available for all projects in GitLab. Usually managed by GitLab administrators and used to execute tasks for different projects.
• Specific Runners — runners tied to a specific project or group of projects. Used for isolation and specialized tasks.
• Group Runners — runners assigned to a group of projects, accessible to all projects within that group.

Each type of runner can be configured to operate in different modes (e.g., shell, docker, docker-machine, etc.), which affects the environment in which tasks are executed.