[tests] allow more tests to run with a container. #817

Job	Run time
shellcheck	11s
Test script (gantry_cleanup_images_spec.sh)	1m 7s
Test script (gantry_common_options_spec.sh)	1m 4s
Test script (gantry_filters_spec.sh)	1m 0s
Test script (gantry_jobs_spec.sh)	44s
Test script (gantry_login_docker_config_spec.sh)	1m 0s
Test script (gantry_login_negative_spec.sh)	52s
Test script (gantry_login_spec.sh)	1m 7s
Test script (gantry_manifest_spec.sh)	47s
Test script (gantry_notify_spec.sh)	1m 20s
Test script (gantry_parallel_spec.sh)	52s
Test script (gantry_rollback_spec.sh)	51s
Test script (gantry_service_multiple_spec.sh)	33s
Test script (gantry_service_no_running_tasks_spec.sh)	47s
Test script (gantry_service_single_spec.sh)	57s
Test script (gantry_update_options_spec.sh)	43s
Build and push Docker image	1m 5s
Test container	2m 54s
	17m 54s

Provide feedback