Document Docker `network_mode` configuration option
From https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/65, the network_mode option is not documented.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
From https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/65, the network_mode option is not documented.