Make selection of subnet using docker more flexible. #2431

Job	Run time
Integration Tests (istioctl, true)	57m 13s
Integration Tests (istioctl, true)	55m 7s
Test Scripts (1.21.x, ubuntu-latest)	4s
Test Scripts (1.21.x, ubuntu-latest)	4s
Unit Tests (1.21.x, ubuntu-latest)	1m 47s
Unit Tests (1.21.x, ubuntu-latest)	1m 47s
Test Scripts (1.21.x, macos-latest)	25s
Test Scripts (1.21.x, macos-latest)	25s
Verify bundle	1m 5s
Verify bundle	1m 5s
Verify fmt	11s
Verify fmt	11s
Verify manifests	33s
Verify manifests	33s
	2h 0m 30s

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