Add `GANTRY_UPDATE_NUM_WORKERS` and other fixes #44

Job	Run time
Run tests (gantry_cleanup_images_spec.sh)	1m 55s
shellcheck	7s
Run tests (gantry_common_options_spec.sh)	1m 30s
Run tests (gantry_filters_spec.sh)	1m 15s
Run tests (gantry_login_spec.sh)	1m 50s
Run tests (gantry_manifest_spec.sh)	1m 36s
Run tests (gantry_notify_spec.sh)	1m 24s
Run tests (gantry_parallel_spec.sh)	2m 6s
Run tests (gantry_rollback_spec.sh)	1m 8s
Run tests (gantry_service_multiple_spec.sh)	1m 36s
Run tests (gantry_service_no_running_tasks_spec.sh)	1m 29s
Run tests (gantry_service_single_spec.sh)	1m 34s
Run tests (gantry_update_options_spec.sh)	1m 44s
	19m 14s

Provide feedback