index.md

Allow User to be specified for a CI Image or Service

Closes gitlab-runner#2750

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137907



Merged-by: Furkan Ayhan <furkanayhn@gmail.com>
Approved-by: Kasia Misirli <kmisirli@gitlab.com>
Approved-by: Mireya Andres <mandres@gitlab.com>
Approved-by: Furkan Ayhan <furkanayhn@gmail.com>
Reviewed-by: Kamil Trzciński <ayufan@ayufan.eu>
Reviewed-by: Avielle Wolfe <awolfe@gitlab.com>
Reviewed-by: Marcel Amirault <mamirault@gitlab.com>
Reviewed-by: Mireya Andres <mandres@gitlab.com>
Reviewed-by: Kasia Misirli <kmisirli@gitlab.com>
Reviewed-by: Muhammed Ali <muhammed.ali@airtimerewards.com>
Co-authored-by: Muhammed Ali <muhammed.ali@airtimerewards.com>
Co-authored-by: Marcel Amirault <mamirault@gitlab.com>
Co-authored-by: Avielle Wolfe <awolfe@gitlab.com>
Co-authored-by: Kasia Misirli <kmisirli@gitlab.com>