[OSPP 2024] optimize image by blob compact

[imeoer]

Nydus can optimize the image size by using chunk dict deduplication, but this can result in small blobs, and poorly written Dockerfile can also result in a lot of small blobs, this affects the I/O read amplification, I/O merge, and data prefetch performance during the image runtime.

Possible implementation steps:

• implement nydusify optimize --policy compact-blob --config opt1=val1,opt2=val2 --target $existed-nydus-image --output $new-nydus-image to generate a new image, and compact the blobs referenced by the nydus image.

[afbjorklund]

Other PRs used the same ("default") also for images and mounts:

images:
- default

mounts:
- default

But those are arrays, so "none" is usually just written out as [].

One could have the string ("none") expand to the empty list?

[afbjorklund]

New syntax:

images: []
mounts: []

base:
- template://_images/ubuntu
- template://_default/mounts