[tests] Add retries for build_test_image. #871

Job	Run time
shellcheck	9s
Test container	2m 58s
Test scripts (gantry_cleanup_images_spec.sh)	1m 7s
Test scripts (gantry_common_options_spec.sh)	1m 11s
Test scripts (gantry_filters_spec.sh)	47s
Test scripts (gantry_jobs_spec.sh)	56s
Test scripts (gantry_login_docker_config_spec.sh)	54s
Test scripts (gantry_login_negative_spec.sh)	58s
Test scripts (gantry_login_spec.sh)	1m 26s
Test scripts (gantry_manifest_spec.sh)	1m 1s
Test scripts (gantry_notify_spec.sh)	1m 28s
Test scripts (gantry_parallel_spec.sh)	1m 1s
Test scripts (gantry_rollback_spec.sh)	54s
Test scripts (gantry_service_multiple_spec.sh)	31s
Test scripts (gantry_service_no_running_tasks_spec.sh)	52s
Test scripts (gantry_service_single_spec.sh)	48s
Test scripts (gantry_update_options_spec.sh)	54s
Build and push Docker image	1m 36s
	19m 31s

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