Run canga/libguestfs inside a docker container

[jfontan]

libguestfs is not always easy to install in all Linux distributions. It's also not available in other OSs.

• Create a docker image with cangallo and libguestfs already installed
• Documentation on how to use the container to manage images
• Add support to cangallo to use the docker image trasparently

[jfontan]

Created a helper shell script to run cangallo in a container instead of adding functionality to cangallo itself.

Execute cangallo in a docker container.

NOTE: The commands can only access files from the current and child
directories as relative paths.

Commands:

* update: download the latest container image
* start: start cangallo container
* stop: stop cangallo container
* any other command is executed by cangallo

Usage:

* Start the container
* Run commands
* Stop the container

dcanga update
dcanga start
dcanga add --copy --tag centos7 CentOS-7-x86_64-GenericCloud-1711.qcow2c
dcanga build centos7.canga
dcanga list
dcanga export -c centos7/example example.qcow2
dcanga stop