[dockerfile] Add coreutils for command `timeout`. #919

Job	Run time
Test container	3m 26s
Test scripts running in parallel	3m 14s
Test scripts (gantry_cleanup_images_spec.sh)	1m 5s
shellcheck	8s
Test scripts (gantry_common_options_spec.sh)	1m 34s
Test scripts (gantry_filters_spec.sh)	50s
Test scripts (gantry_jobs_spec.sh)	40s
Test scripts (gantry_login_docker_config_spec.sh)	1m 3s
Test scripts (gantry_login_negative_spec.sh)	1m 4s
Test scripts (gantry_login_spec.sh)	1m 11s
Test scripts (gantry_manifest_spec.sh)	1m 4s
Test scripts (gantry_notify_spec.sh)	1m 17s
Test scripts (gantry_parallel_spec.sh)	1m 1s
Test scripts (gantry_rollback_spec.sh)	48s
Test scripts (gantry_service_multiple_spec.sh)	32s
Test scripts (gantry_service_no_running_tasks_spec.sh)	52s
Test scripts (gantry_service_single_spec.sh)	57s
Test scripts (gantry_update_options_spec.sh)	59s
Build and push Docker image	1m 17s
	23m 2s

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