[tests] Add retries for build_test_image. #869

Job	Run time
Test container	2m 57s
shellcheck	10s
Test scripts (gantry_cleanup_images_spec.sh)	57s
Test scripts (gantry_common_options_spec.sh)	1m 4s
Test scripts (gantry_filters_spec.sh)	47s
Test scripts (gantry_jobs_spec.sh)	47s
Test scripts (gantry_login_docker_config_spec.sh)	54s
Test scripts (gantry_login_negative_spec.sh)	1m 2s
Test scripts (gantry_login_spec.sh)	1m 9s
Test scripts (gantry_manifest_spec.sh)	1m 3s
Test scripts (gantry_notify_spec.sh)	1m 12s
Test scripts (gantry_parallel_spec.sh)	56s
Test scripts (gantry_rollback_spec.sh)	45s
Test scripts (gantry_service_multiple_spec.sh)	41s
Test scripts (gantry_service_no_running_tasks_spec.sh)	54s
Test scripts (gantry_service_single_spec.sh)	49s
Test scripts (gantry_update_options_spec.sh)	56s
Build and push Docker image	1m 3s
	18m 6s

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