add support to import image from docker-daemon in rkt

[surajssd]

Add support to import image from docker-daemon in rkt

Now you can import an image that is available in docker's image store on
a local machine into rkt's imagestore.

To do so run following command:

rkt fetch --insecure-options=image docker-daemon://docker.io/library/alpine

or directly run it the usual way

rkt run --insecure-options=image docker-daemon://docker.io/library/alpine

This will import the image docker.io/library/alpine into rkt's store.

[surajssd]

This PR first fetches the image from machine's running docker daemon into a tar format. This is similar to running a docker save.

Then it is converted into aci format and then imported into the rkt's image local store.

[alban]

aciCleaner() called twice? (line 45 and 52)

[alban]

oops, I can't read. ignore me

[onlyjob]

That's a nice change to automatically invoke equivalent of two commands (docker image save, docker2aci) in order to convert docker container image from local store. It works as expected and makes rkt slightly more intuitive when it comes to first import of a foreign container image from Docker.

However I recommend not to merge it as it is, for numerous reasons:

1. This change belongs to docker2aci which should support similar syntax.
   However there is appc->oci: get rid of docker2aci  #3378 hence the proper place to implement it would be in github.com/containers/image.

2. In order to work this change requires Docker daemon to be running. But Docker is selfish and hostile: currently it unconditionally modifies iptables rules in a way that breaks networking in rkt. At least at the moment (and already for a while because Docker couldn't care less) Docker and rkt can not be used together.

3. My biggest concern is about introducing competitor as a direct dependency.
   This is not just a strategic concern but also a practical problem because it bloats rkt's dependency graph with 100+ libraries, dramatically increasing surface for bugs.
   Apparently that's not only my concern: Replace github.com/docker/docker/client dependency? containers/image#476
   Downstream in Debian where we build software from fully tested libraries, bugs in Docker dependency tree are regularly exposed - that makes maintenance of Docker very tedious. rkt is much better in that regards due to much smaller dependency tree.