[lib-gantry] add GANTRY_NOTIFICATION_CONDITION #424

Job	Run time
shellcheck	15s
Test script (gantry_cleanup_images_spec.sh)	2m 24s
Test script (gantry_common_options_spec.sh)	1m 26s
Test script (gantry_filters_spec.sh)	1m 42s
Test script (gantry_login_spec.sh)	2m 10s
Test script (gantry_manifest_spec.sh)	1m 35s
Test script (gantry_notify_spec.sh)	1m 45s
Test script (gantry_parallel_spec.sh)	2m 7s
Test script (gantry_rollback_spec.sh)	59s
Test script (gantry_service_multiple_spec.sh)	1m 35s
Test script (gantry_service_no_running_tasks_spec.sh)	1m 38s
Test script (gantry_service_single_spec.sh)	1m 32s
Test script (gantry_update_options_spec.sh)	1m 33s
Build and push Docker image	1m 12s
Test container	2m 19s
	24m 12s

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