Implementation of CNB Image interface using OCI layout format

[jjbustamante]

Summary

This pull request implements the CNB Image interface using OCI layout format.

A new layout package was added to expose the implementation of the Image interface

The implementation exposes the following classes:

classDiagram
class ImageOption {
    <<interface>>
    FromBaseImage(base v1.Image) ImageOption
    FromBaseImagePath(path string) ImageOption
    WithPreviousImage(path string)
}
class LayoutImage {
    NewImage(path string, ops ...ImageOption) (*Image, error)
}
class SparseImage {
     NewImage(path string, from v1.Image) (*Image, error) 
}

Where

• LayoutImage: is actually layout.Image but mermaid doesn't allow to define the package name. Implements the Image interface and handle the complexity of saving on Disk in OCI layout format.
• SparseImage: sparse.Image is an alternative implementation of the Image interface that mimics the behavior of a image without actually saving any layer on disk.

Output

When the image.Save() method is called it will save the output in the local filesystem at the path provided during initialization in OCI layout format, for example:

The previous image shows a couple of examples of two images save in OCI layout format on disk

Before

It was not possible to save images in OCI layout format

After

New package layout is exposed with the new implementation.

Notes

• This is an experimental feature
• The following methods will not be implemented in this iteration
  • image.Rebase()
  • image.SaveFile()

Fixes #162

Signed-off-by: Juan Bustamante jbustamante@vmware.com

[natalieparellano]

@jjbustamante added a few nits... I had some questions in my previous set of comments that would be good to answer, but overall I'm feeling really good about this PR :)

[jjbustamante]

Hi @jabrown85

If you can take a look and provide some feedback will be awesome! I already went to some of the comments Natalie left me.