[tests] allow more tests to run with a container. #788

Job	Run time
shellcheck	9s
Test script (gantry_cleanup_images_spec.sh)	1m 6s
Test script (gantry_common_options_spec.sh)	1m 5s
Test script (gantry_filters_spec.sh)	54s
Test script (gantry_jobs_spec.sh)	44s
Test script (gantry_login_docker_config_spec.sh)	1m 19s
Test script (gantry_login_negative_spec.sh)	1m 3s
Test script (gantry_login_spec.sh)	1m 5s
Test script (gantry_manifest_spec.sh)	52s
Test script (gantry_notify_spec.sh)	1m 21s
Test script (gantry_parallel_spec.sh)	1m 10s
Test script (gantry_rollback_spec.sh)	48s
Test script (gantry_service_multiple_spec.sh)	41s
Test script (gantry_service_no_running_tasks_spec.sh)	1m 1s
Test script (gantry_service_single_spec.sh)	45s
Test script (gantry_update_options_spec.sh)	58s
Build and push Docker image	1m 11s
Test container	3m 4s
	19m 16s

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