[tests] allow more tests to run with a container. #815

Job	Run time
shellcheck	7s
Test script (gantry_cleanup_images_spec.sh)	1m 4s
Test script (gantry_common_options_spec.sh)	1m 8s
Test script (gantry_filters_spec.sh)	57s
Test script (gantry_jobs_spec.sh)	40s
Test script (gantry_login_docker_config_spec.sh)	49s
Test script (gantry_login_negative_spec.sh)	55s
Test script (gantry_login_spec.sh)	58s
Test script (gantry_manifest_spec.sh)	1m 0s
Test script (gantry_notify_spec.sh)	1m 9s
Test script (gantry_parallel_spec.sh)	53s
Test script (gantry_rollback_spec.sh)	51s
Test script (gantry_service_multiple_spec.sh)	34s
Test script (gantry_service_no_running_tasks_spec.sh)	51s
Test script (gantry_service_single_spec.sh)	49s
Test script (gantry_update_options_spec.sh)	44s
Build and push Docker image	59s
Test container	2m 49s
	17m 17s

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