hostinfo: detect AWS Lambda as a container.

AWS Lambda uses Docker containers but does not
have the string "docker" in its /proc/1/cgroup.
Infer AWS Lambda via the environment variables
it sets.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

control/controlclient/hostinfo_linux.go

@@ -62,6 +62,9 @@ func osVersionLinux() string {
 	if inKnative() {
 		attrBuf.WriteString("; env=kn")
 	}
+	if inAwsLambda() {
+		attrBuf.WriteString("; env=lm")
+	}
 	attr := attrBuf.String()
 
 	id := m["ID"]
@@ -121,3 +124,14 @@ func inKnative() bool {
 	}
 	return false
 }
+
+func inAwsLambda() bool {
+	// https://docs.aws.amazon.com/lambda/latest/dg/configuration-envvars.html
+	if os.Getenv("AWS_LAMBDA_FUNCTION_NAME") != "" &&
+		os.Getenv("AWS_LAMBDA_FUNCTION_VERSION") != "" &&
+		os.Getenv("AWS_LAMBDA_INITIALIZATION_TYPE") != "" &&
+		os.Getenv("AWS_LAMBDA_RUNTIME_API") != "" {
+		return true
+	}
+	return false
+}