allow for custom domain in k8s-nameserver #12

Job	Run time
race-root-integration (1/4)	1m 19s
race-root-integration (2/4)	1m 20s
race-root-integration (3/4)	1m 18s
race-root-integration (4/4)	1m 21s
test (amd64, -coverprofile=/tmp/coverage.out)	3m 31s
test (amd64, -race, 1/3)	3m 24s
windows	4m 9s
test (amd64, -race, 2/3)	3m 2s
test (amd64, -race, 3/3)	3m 40s
test (386)	2m 41s
cross (linux, arm64)	1m 24s
ios	46s
cross (linux, 386)	1m 23s
privileged	53s
cross (linux, loong64)	1m 31s
race-build	4m 33s
cross (linux, arm, 5)	1m 11s
vm	0s
cross (linux, arm, 7)	2m 21s
cross (darwin, amd64)	1m 24s
android	55s
cross (darwin, arm64)	1m 18s
crossmin (plan9, amd64)	21s
crossmin (aix, ppc64)	18s
cross (windows, amd64)	1m 4s
tailscale_go	30s
cross (windows, arm64)	1m 3s
cross (freebsd, amd64)	1m 11s
wasm	6m 33s
depaware	33s
fuzz	7m 6s
go_generate	1m 7s
cross (openbsd, amd64)	1m 19s
go_mod_tidy	28s
licenses	6s
staticcheck (linux, amd64)	3m 39s
staticcheck (windows, amd64)	3m 29s
staticcheck (darwin, amd64)	3m 29s
staticcheck (windows, 386)	3m 22s
notify_slack	2s
check_mergeability	7s
	1h 19m 11s

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