[tests] allow more tests to run with a container. #794

Job	Run time
Test script (gantry_cleanup_images_spec.sh)	1m 3s
shellcheck	9s
Test script (gantry_common_options_spec.sh)	1m 14s
Test script (gantry_filters_spec.sh)	41s
Test script (gantry_jobs_spec.sh)	36s
Test script (gantry_login_docker_config_spec.sh)	52s
Test script (gantry_login_negative_spec.sh)	1m 11s
Test script (gantry_login_spec.sh)	1m 0s
Test script (gantry_manifest_spec.sh)	1m 2s
Test script (gantry_notify_spec.sh)	1m 8s
Test script (gantry_parallel_spec.sh)	1m 2s
Test script (gantry_rollback_spec.sh)	56s
Test script (gantry_service_multiple_spec.sh)	32s
Test script (gantry_service_no_running_tasks_spec.sh)	56s
Test script (gantry_service_single_spec.sh)	59s
Test script (gantry_update_options_spec.sh)	55s
Build and push Docker image	58s
Test container	2m 43s
	17m 57s

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