Tidy GetDockerOS() function

Signed-off-by: John Howard <jhoward@microsoft.com>

client/utils.go

@@ -0,0 +1,15 @@
+package client
+
+import "regexp"
+
+var headerRegexp = regexp.MustCompile(`\ADocker/.+\s\((.+)\)\z`)
+
+// getDockerOS returns the operating system based on the server header from the daemon.
+func getDockerOS(serverHeader string) string {
+	var osType string
+	matches := headerRegexp.FindStringSubmatch(serverHeader)
+	if len(matches) > 0 {
+		osType = matches[1]
+	}
+	return osType
+}