main.go

package main

import (
	"fmt"
	"os"
	"os/exec"
	"syscall"
)

// docker run container-name cmd args
// go run main.go run cmd args
func main() {
	switch os.Args[1] {
	case "run":
		run();
	case "child":
		child()
	default:
		panic("Unknown command!");
	}
}

func run() {

	cmd := exec.Command("/proc/self/exe", append([]string{"child"}, os.Args[2:]...)...)
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	cmd.SysProcAttr = &syscall.SysProcAttr {
		Cloneflags: syscall.CLONE_NEWUTS | syscall.CLONE_NEWPID,
	}

	must(cmd.Run())
}

func child() {
	fmt.Printf("PID: %d :: Inside container running command: [%v]\n", os.Getpid(), os.Args[2:])

	cmd := exec.Command(os.Args[2], os.Args[3:]...)
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	must(syscall.Sethostname([]byte("container")))
	must(syscall.Chroot("/rootfs-ubuntu"))
	must(os.Chdir("/"))
	must(syscall.Mount("proc", "proc", "proc", 0, ""))

	must(cmd.Run())
	
	must(syscall.Unmount("proc", 0))
}

func must(err error) {
	if(err != nil) {
		panic(err)
	}
}