[proposal] support loading docker format image

[chuanchang]

Ⅰ. Issue Description

At present, it's incompatible to load docker save image from pouch, perhaps, a --format option can be added into pouch load command, and OCI image is a default, if users set --format docker, and then convert docker image to oci image automatically.

Ⅱ. Describe what happened

#pouch load -i busybox.tar
Error: {"message":"failed to import image into containerd by tarstream: no index.json found"}

Ⅲ. Describe what you expected to happen

Pouch can load docker format image

Ⅳ. How to reproduce it (as minimally and precisely as possible)

1. docker pull $image
2. docker save $image -o $image.tar
3. pouch load -i $image.tar

Ⅴ. Anything else we need to know?

A tool convert docker image to oci image: https://github.com/coolljt0725/docker2oci
Please reference PR: #1163

Ⅵ. Environment:

• pouch version (use pouch version):
  $ pouch version
  BuildTime: 2018-07-26T11:10:46+08:00
  GitCommit: 1.0.0.rc1-53-gb469e1e
  GoVersion: go1.9.4
  KernelVersion: 3.10.0-693.2.2.el7.x86_64
  Os: linux
  Version: 1.0.0-rc1
  APIVersion: 1.24
  Arch: amd64

• OS (e.g. from /etc/os-release):
  $ cat /etc/os-release
  NAME="CentOS Linux"
  VERSION="7 (Core)"
  ID="centos"
  ID_LIKE="rhel fedora"
  VERSION_ID="7"
  PRETTY_NAME="CentOS Linux 7 (Core)"
  ANSI_COLOR="0;31"
  CPE_NAME="cpe:/o:centos:centos:7"
  HOME_URL="https://www.centos.org/"
  BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

• Kernel (e.g. uname -a):
  $ uname -a
  Linux izj6cfln087djyclspbpngz 3.10.0-693.2.2.el7.x86_64 add travis.yml for CI #1 SMP Tue Sep 12 22:26:13 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

• Install tools:
  docker

• Others:

[allencloud]

What is the status of this issue? @chuanchang @xiechengsheng @fuweid

[xiechengsheng]

In fact moby project is working on this in this issue and Pull Request, seems very large feature in moby...