[tests] fix shellcheck. #917

Job	Run time
shellcheck	12s
Test scripts (gantry_cleanup_images_spec.sh)	1m 17s
Test scripts (gantry_common_options_spec.sh)	1m 38s
Test scripts (gantry_filters_spec.sh)	52s
Test scripts (gantry_jobs_spec.sh)	41s
Test scripts (gantry_login_docker_config_spec.sh)	1m 4s
Test container	3m 5s
Test scripts (gantry_login_negative_spec.sh)	59s
Test scripts running in parallel	3m 28s
Test scripts (gantry_login_spec.sh)	1m 0s
Test scripts (gantry_manifest_spec.sh)	52s
Test scripts (gantry_notify_spec.sh)	1m 19s
Test scripts (gantry_parallel_spec.sh)	58s
Test scripts (gantry_rollback_spec.sh)	50s
Test scripts (gantry_service_multiple_spec.sh)	34s
Test scripts (gantry_service_no_running_tasks_spec.sh)	51s
Test scripts (gantry_service_single_spec.sh)	58s
Test scripts (gantry_update_options_spec.sh)	55s
Build and push Docker image	1m 22s
	22m 55s

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