"library image" support for "docker.io" "index.docker.io"

[xujinheng]

What would you like to be added:

This is how imageswap determine whether a image is a library image now:

if image_split[1] == "" and image_split[2] == "":

It can deal with cases like "nginx", but can not detect "docker.io/nginx" and "index.docker.io/nginx" as library images. Because:

>>> "nginx".partition("/")
('nginx', '', '')
>>> "docker.io/nginx".partition("/")
('docker.io', '/', 'nginx')
>>> "index.docker.io/nginx".partition("/")
('index.docker.io', '/', 'nginx')

Why is this needed: To cover more cases when input images are from first level repositories

[justusbunsi]

This is already possible with the correct replacement mappings.

docker.io/library::<your registry>
index.docker.io/library::<your registry>

Tested with 1.5.2.