hostinfo: add EnvType for Kubernetes

Signed-off-by: Maisem Ali <maisem@tailscale.com>
(cherry picked from commit 2662a1c98c)

hostinfo/hostinfo.go

@@ -82,6 +82,7 @@ const (
 	AzureAppService = EnvType("az")
 	AWSFargate      = EnvType("fg")
 	FlyDotIo        = EnvType("fly")
+	Kubernetes      = EnvType("k8s")
 )
 
 var envType atomic.Value // of EnvType
@@ -136,6 +137,9 @@ func getEnvType() EnvType {
 	if inFlyDotIo() {
 		return FlyDotIo
 	}
+	if inKubernetes() {
+		return Kubernetes
+	}
 	return ""
 }
 
@@ -212,3 +216,10 @@ func inFlyDotIo() bool {
 	}
 	return false
 }
+
+func inKubernetes() bool {
+	if os.Getenv("KUBERNETES_SERVICE_HOST") != "" && os.Getenv("KUBERNETES_SERVICE_PORT") != "" {
+		return true
+	}
+	return false
+}