ensure enough loop devices. Pulled from https://github.com/jpetazzo/dind/issues/19#issuecomment-48859883 but ensure there are at least 4 loop devices

ensure_loops.sh

#!/bin/bash
ensure_loop(){
  num="$1"
  dev="/dev/loop$num"
  if test -b "$dev"; then
    echo "$dev is a usable loop device."
    return 0
  fi

  echo "Attempting to create $dev for docker ..."
  if ! mknod -m660 $dev b 7 $num; then
    echo "Failed to create $dev!" 1>&2
    return 3
  fi

  return 0
}

LOOP_A=$(losetup -f)
LOOP_A=${LOOP_A#/dev/loop}
LOOP_B=$(expr $LOOP_A + 1)
LOOP_C=$(expr $LOOP_B + 1)
LOOP_D=$(expr $LOOP_C + 1)

ensure_loop $LOOP_A
ensure_loop $LOOP_B
ensure_loop $LOOP_C
ensure_loop $LOOP_D