[dockerfile] Add coreutils for command `timeout`. #920

Job	Run time
shellcheck	10s
Test container	3m 28s
Test scripts (gantry_cleanup_images_spec.sh)	1m 7s
Test scripts (gantry_common_options_spec.sh)	1m 40s
Test scripts (gantry_filters_spec.sh)	56s
Test scripts (gantry_jobs_spec.sh)	53s
Test scripts (gantry_login_docker_config_spec.sh)	1m 5s
Test scripts (gantry_login_negative_spec.sh)	1m 2s
Test scripts running in parallel	3m 23s
Test scripts (gantry_login_spec.sh)	1m 13s
Test scripts (gantry_manifest_spec.sh)	56s
Test scripts (gantry_notify_spec.sh)	1m 20s
Test scripts (gantry_parallel_spec.sh)	1m 1s
Test scripts (gantry_rollback_spec.sh)	49s
Test scripts (gantry_service_multiple_spec.sh)	35s
Test scripts (gantry_service_no_running_tasks_spec.sh)	53s
Test scripts (gantry_service_single_spec.sh)	1m 8s
Test scripts (gantry_update_options_spec.sh)	1m 5s
Build and push Docker image	1m 41s
	24m 25s

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