Fix infinite recursion in buildImage with default tag #573

Job	Run time
lint (golangci-lint)	4s
build (nix-snapshotter)	1m 28s
build (image-hello)	1m 33s
build (image-redis)	2m 22s
build (image-redisWithShell)	1m 41s
integration-test (snapshotter)	4m 1s
integration-test (kubernetes)	5m 47s
integration-test (k3s)	12m 54s
integration-test (k3s-external)	14m 59s
integration-test (k3s-rootless)	12m 35s
integration-test (gvisor)	3m 51s
coverage-report	53s
	1h 2m 8s

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