Remove CGO dependency for user lookup in allocdir

os/user's user.Lookup requires that the artifact be compiled with CGO
support enabled. This change instead reads /etc/passwd directly.

The code was acquired from docker/docker#1096

client/allocdir/alloc_dir_posix.go

@@ -5,9 +5,11 @@ package allocdir
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
 	"os/user"
 	"strconv"
+	"strings"
 	"syscall"
 )
 
@@ -26,7 +28,7 @@ func (d *AllocDir) dropDirPermissions(path string) error {
 		return nil
 	}
 
-	u, err := user.Lookup("nobody")
+	u, err := userLookup("nobody")
 	if err != nil {
 		return err
 	}
@@ -69,3 +71,28 @@ func getGid(u *user.User) (int, error) {
 
 	return gid, nil
 }
+
+// userLookup checks if the given username or uid is present in /etc/passwd
+// and returns the user struct.
+// If the username is not found, an error is returned.
+// Credit to @creak, https://github.com/docker/docker/pull/1096
+func userLookup(uid string) (*user.User, error) {
+	file, err := ioutil.ReadFile("/etc/passwd")
+	if err != nil {
+		return nil, err
+	}
+
+	for _, line := range strings.Split(string(file), "\n") {
+		data := strings.Split(line, ":")
+		if len(data) > 5 && (data[0] == uid || data[2] == uid) {
+			return &user.User{
+				Uid:      data[2],
+				Gid:      data[3],
+				Username: data[0],
+				Name:     data[4],
+				HomeDir:  data[5],
+			}, nil
+		}
+	}
+	return nil, fmt.Errorf("User not found in /etc/passwd")
+}