[tests] allow more tests to run with a container. #811

Job	Run time
shellcheck	11s
Test script (gantry_cleanup_images_spec.sh)	1m 0s
Test script (gantry_common_options_spec.sh)	1m 2s
Test script (gantry_filters_spec.sh)	41s
Test script (gantry_jobs_spec.sh)	36s
Test script (gantry_login_docker_config_spec.sh)	58s
Test script (gantry_login_negative_spec.sh)	1m 5s
Test script (gantry_login_spec.sh)	57s
Test script (gantry_manifest_spec.sh)	1m 13s
Test script (gantry_notify_spec.sh)	1m 30s
Test script (gantry_parallel_spec.sh)	1m 1s
Test script (gantry_rollback_spec.sh)	56s
Test script (gantry_service_multiple_spec.sh)	27s
Test script (gantry_service_no_running_tasks_spec.sh)	51s
Test script (gantry_service_single_spec.sh)	43s
Test script (gantry_update_options_spec.sh)	44s
Build and push Docker image	1m 2s
Test container	3m 24s
	18m 21s

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