[tests] allow more tests to run with a container. #775

Job	Run time
shellcheck	8s
Test script (gantry_cleanup_images_spec.sh)	1m 0s
Test script (gantry_common_options_spec.sh)	1m 13s
Test script (gantry_filters_spec.sh)	45s
Test script (gantry_jobs_spec.sh)	49s
Test script (gantry_login_docker_config_spec.sh)	52s
Test script (gantry_login_negative_spec.sh)	58s
Test script (gantry_login_spec.sh)	1m 10s
Test script (gantry_manifest_spec.sh)	52s
Test script (gantry_notify_spec.sh)	1m 10s
Test script (gantry_parallel_spec.sh)	1m 7s
Test script (gantry_rollback_spec.sh)	45s
Test script (gantry_service_multiple_spec.sh)	31s
Test script (gantry_service_no_running_tasks_spec.sh)	56s
Test script (gantry_service_single_spec.sh)	44s
Test script (gantry_update_options_spec.sh)	59s
Build and push Docker image	1m 29s
Test container	2m 24s
	17m 52s

Provide feedback