Bump alpine from 3.20.3 to 3.21.0 #103

Job	Run time
shellcheck	9s
Test container	3m 29s
Test scripts (gantry_cleanup_images_spec.sh)	59s
Test scripts (gantry_common_options_spec.sh)	1m 31s
Test scripts (gantry_filters_spec.sh)	59s
Test scripts running in parallel	3m 17s
Test scripts (gantry_jobs_spec.sh)	40s
Test scripts (gantry_login_docker_config_spec.sh)	53s
Test scripts (gantry_login_negative_spec.sh)	1m 5s
Test scripts (gantry_login_spec.sh)	1m 1s
Test scripts (gantry_manifest_spec.sh)	55s
Test scripts (gantry_notify_spec.sh)	1m 21s
Test scripts (gantry_parallel_spec.sh)	1m 2s
Test scripts (gantry_rollback_spec.sh)	49s
Test scripts (gantry_service_multiple_spec.sh)	34s
Test scripts (gantry_service_no_running_tasks_spec.sh)	1m 1s
Test scripts (gantry_service_single_spec.sh)	1m 7s
Test scripts (gantry_update_options_spec.sh)	55s
	21m 47s

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