Clean out untagged Docker images after build

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -228,6 +229,29 @@ func main() {
 		}
 	}
 
+	// Remove untagged images, if any
+	var outbuf bytes.Buffer
+	cmd = exec.Command("sh", "-c", "docker images | grep '^<none>' | awk '{print $3}'")
+	cmd.Stdout = &outbuf
+	cmd.Stderr = os.Stderr
+	trace(cmd)
+	err = cmd.Run()
+	if err != nil {
+		os.Exit(1)
+	}
+
+	if outbuf.Len() > 0 {
+		images := strings.Split(strings.TrimSpace(outbuf.String()), "\n")
+		cmd = exec.Command("docker", append([]string{"rmi"}, images...)...)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		trace(cmd)
+		err := cmd.Run()
+		if err != nil {
+			os.Exit(1)
+		}
+	}
+
 	// Save to tarred image repository
 	if len(vargs.Save.File) != 0 {
 		// if the destination directory does not exist, create it