main.go

package main

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

// docker run <container> cmd args
// go run main.go run cmd args
func main() {
  switch os.Args[1] {
  case "run":
    run()
  case "child":
    child()
  default:
    panic("Syntax Error!\ngo run main.go run <cmd> <args>")
  }
}

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 | syscall.CLONE_NEWNS | syscall.CLONE_NEWUSER,
    Credential: &syscall.Credential{Uid: 0, Gid: 0},
    UidMappings: []syscall.SysProcIDMap{
      {ContainerID: 0, HostID: os.Getuid(), Size: 1},
    },
    GidMappings: []syscall.SysProcIDMap{
      {ContainerID: 0, HostID: os.Getgid(), Size: 1},
    },
  }

  must(cmd.Run())
}

func child() {
  fmt.Printf("running %v as PID %d\n ", os.Args[2:], os.Getpid())
  cmd := exec.Command(os.Args[2], os.Args[3:]...)
  cmd.Stdin = os.Stdin
  cmd.Stdout = os.Stdout
  cmd.Stderr = os.Stderr

  must(syscall.Chroot("/data/Projects/containers_from_scratch/new_fs"))
  must(syscall.Sethostname([]byte("container")))
  must(os.Chdir("/"))
  must(syscall.Mount("proc", "proc", "proc", 0, ""))
  must(cmd.Run())
}

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