main.go

package main

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

func main() {
	switch os.Args[1] {
	case "run":
		run()
	case "child":
		child()

	default:
		panic("bad command")
	}
}

func run() {
	// cmd struct to execute named program with given arguments
	cmd := exec.Command("/proc/self/exe", append([]string{"child"}, os.Args[2:]...)...)
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	// operating system specific attributes - run passes it to new process as sys fields
	cmd.SysProcAttr = &syscall.SysProcAttr{
		Cloneflags:   syscall.CLONE_NEWUTS | syscall.CLONE_NEWPID | syscall.CLONE_NEWNS,
		Unshareflags: syscall.CLONE_NEWNS,
	}

	must(cmd.Run())
}

func child() {
	fmt.Printf("Running %v as process %d inside container\n", os.Args[2:], os.Getpid())
	fmt.Printf("-------------------------------------------------------------\n")

	syscall.Sethostname([]byte(os.Args[3]))

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

	must(cmd.Run())
}

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