new command: crane size

[imjasonh]

This adds crane size which returns the total compressed size of blobs referenced by a manifest. This can be useful to roughly determine the relative size of two images in a registry, and over the wire.

This won't tell you how many uncompressed bytes the image might take up if you pull it locally.

go run ./cmd/crane size ubuntu                               
146051085

go run ./cmd/crane size ubuntu --human                       
139 MiB

go run ./cmd/crane size ubuntu --platform=linux/arm64
27361037

go run ./cmd/crane size ubuntu --platform=linux/ppc64le
34496056

go run ./cmd/crane size ubuntu --platform=linux/s390x --human
27 MiB

It forks some code from https://github.com/dustin/go-humanize/blob/master/bytes.go which hasn't been changed in 7 years, so it's expected to be pretty stable. This avoids taking a full dependency on the package, which overall does get occasional updates.

Open to ideas about other things we may want to support with this. This is a Go port of this one-liner that I use pretty often:

function imgsize() {
  crane manifest $1 --platform ${2:-linux/amd64} | jq '.config.size + ([.layers[].size] | add)' | numfmt --to=iec
}