[tests] allow more tests to run with a container. #812

Job	Run time
shellcheck	9s
Test script (gantry_cleanup_images_spec.sh)	1m 6s
Test script (gantry_common_options_spec.sh)	1m 11s
Test script (gantry_filters_spec.sh)	42s
Test script (gantry_jobs_spec.sh)	38s
Test script (gantry_login_docker_config_spec.sh)	59s
Test script (gantry_login_negative_spec.sh)	1m 10s
Test script (gantry_login_spec.sh)	1m 6s
Test script (gantry_manifest_spec.sh)	53s
Test script (gantry_notify_spec.sh)	1m 13s
Test script (gantry_parallel_spec.sh)	53s
Test script (gantry_rollback_spec.sh)	42s
Test script (gantry_service_multiple_spec.sh)	27s
Test script (gantry_service_no_running_tasks_spec.sh)	47s
Test script (gantry_service_single_spec.sh)	45s
Test script (gantry_update_options_spec.sh)	48s
Build and push Docker image	59s
Test container	3m 6s
	17m 34s

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