Make docker permissions errors more obvious

main.go

@@ -61,6 +61,14 @@ func checkRequiredUtils() (err error) {
 	return
 }
 
+func checkDockerPermissions() (err error) {
+	output, err := exec.Command("docker", "ps").CombinedOutput()
+	if err != nil {
+		err = fmt.Errorf(string(output))
+	}
+	return
+}
+
 func main() {
 	app := kingpin.New(
 		"out-of-tree",
@@ -140,6 +148,15 @@ func main() {
 		log.Fatalln(err)
 	}
 
+	err = checkDockerPermissions()
+	if err != nil {
+		log.Println(err)
+		log.Println("You have two options:")
+		log.Println("\t1. Add user to group docker;")
+		log.Println("\t2. Run out-of-tree with sudo.")
+		os.Exit(1)
+	}
+
 	if !exists(usr.HomeDir + "/.out-of-tree/images") {
 		log.Println("No ~/.out-of-tree/images: " +
 			"Probably you need to run `out-of-tree bootstrap`" +