WIP: Add Skopeo source

[guillaumerose]

Fixes #130

The main benefit of this source is that it doesn't need a daemon.

For instance, on macOS, just run:

$ brew install skopeo
$ dive skopeo://redis 
Image Source: skopeo://redis
Fetching image... (this can take a while for large images)
Getting image source signatures
Copying blob 852e50cd189d done  
Copying blob 76190fa64fb8 done  
Copying blob 9cbb1b61e01b done  
Copying blob d048021f2aae done  
Copying blob 6f4b2af24926 done  
Copying blob 1cf1d6922fba done  
Copying config 74d1072210 done  
Writing manifest to image destination
Storing signatures
Analyzing image...
Building cache...

I wanted to browse an image on my Mac. This was the easiest solution to do it without having a daemon running :)

TODO:

• decide wether code reading the tar layer should be extracted to a new package shared by Docker and Skopeo.
• handle extra flags for Skopeo (--override-os=linux is most of the time mandatory)
• add .exe extension for Windows and check if it works.

Let me know what you think about this!

[guillaumerose]

I just discovered the docker-archive format output of skopeo. I will use it.

[guillaumerose]

It's maybe easier to do it in 2 commands. Will re-open if needed.