Add documentation for CI_JOB_IMAGE environment var

Introduced in https://gitlab.com/gitlab-org/gitlab-runner/issues/4888

doc/ci/variables/predefined_variables.md

@@ -57,6 +57,7 @@ future GitLab releases.**
 | `CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_NAME` | 12.3   | all    | The target branch name of the pull request if [the pipelines are for external pull requests](../ci_cd_for_external_repos/index.md#pipelines-for-external-pull-requests). Available only if `only: [external_pull_requests]` is used and the pull request is open.                                                                                          |
 | `CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_SHA`  | 12.3   | all    | The HEAD SHA of the target branch of the pull request if [the pipelines are for external pull requests](../ci_cd_for_external_repos/index.md#pipelines-for-external-pull-requests). Available only if `only: [external_pull_requests]` is used and the pull request is open.                                                                               |
 | `CI_JOB_ID`                                   | 9.0    | all    | The unique id of the current job that GitLab CI uses internally                                                                                                                                                                                                                                                                                            |
+| `CI_JOB_IMAGE`                                | 12.9   | 12.9   | The name of the image running the CI job                                                                                                                                                                                                                                                                                                                   |
 | `CI_JOB_MANUAL`                               | 8.12   | all    | The flag to indicate that job was manually started                                                                                                                                                                                                                                                                                                         |
 | `CI_JOB_NAME`                                 | 9.0    | 0.5    | The name of the job as defined in `.gitlab-ci.yml`                                                                                                                                                                                                                                                                                                         |
 | `CI_JOB_STAGE`                                | 9.0    | 0.5    | The name of the stage as defined in `.gitlab-ci.yml`                                                                                                                                                                                                                                                                                                       |