Getting an image's digest fast, within a docker-container builder

[fenollp]

Hi!

I've tried both:

• BUILDX_BUILDER=supergreen docker buildx imagetools inspect docker.io/library/rust:1.89.0-slim '--format={{json .Manifest.Digest}}'

• BUILDX_BUILDER=supergreen docker manifest inspect docker.io/library/rust:1.89.0-slim

to get that image's digest from my builder's cache (it's there, all local). BTW the expected answer (today) is:

"sha256:33219ca58c0dd38571fd3f87172b5bce2d9f3eb6f27e6e75efe12381836f71fa"

However it is incomprehensible to me why even just the (non-experimental) imagetools inspect command takes 4-5s to respond... I don't see any CPU/RAM spikes... Is this hitting the network? Everything should be in my builder's cache though.

How can I get the digest of my local copy of a tag in <1s?

Oh and BTW --format={{.Manifest.Digest}} (without json formatting) is still broken: it ignores the Go tmpl.

Thanks!

[fenollp]

To be clear, this command used to work (with the old Docker --loaded images): and was fast

BUILDX_BUILDER=supergreen docker --debug inspect --'format={{index .RepoDigests 0}}' docker.io/library/rust:1.89.0-slim

More on the imagetools command: with --debug it shows this

DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: resolving
DEBUG: do request
DEBUG: fetch response received
DEBUG: resolved
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: fetch
DEBUG: fetch
DEBUG: fetch
DEBUG: fetch
DEBUG: fetch
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch
DEBUG: fetching layer
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetching layer
DEBUG: fetching layer
DEBUG: do request
DEBUG: do request
DEBUG: fetch
DEBUG: fetching layer
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: do request
DEBUG: fetch
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch
DEBUG: do request
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch response received
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received
DEBUG: fetch
DEBUG: fetching layer
DEBUG: do request
DEBUG: fetch response received

and today, gives me a different response than yesterday:

"sha256:2ff54dd21007d5ee97026fadad80598e66136a43adc5687078d796d958bd58fb"

which tells me this command hits the network.
I'm looking for a way to read image digest from my /local/ builder cache though.

[fenollp]

Adding to my question, I found moby/buildkit#4854 which is IMO a bit more general than what I'm asking for here:

I'm trying to get the manifest digest of an image in my builder cache, but that image isn't built here: it's from a registry. E.g.: rust:1.89.0-slim
In my case maybe there's a simple/quick workaround? I actually found this one:

docker buildx du --verbose | grep rust:1.89.0-slim | sort -u

Description: pulled from docker.io/library/rust:1.89.0-slim@sha256:33219ca58c0dd38571fd3f87172b5bce2d9f3eb6f27e6e75efe12381836f71fa

which appears to match

ID:		ohxhekyoshxip5l5hnd3th9jb
Parent:		dyoo0ez6aq47esc1lu7gij20a
Created at:	2025-08-12 13:04:40.701102099 +0000 UTC
Mutable:	false
Reclaimable:	true
Shared:		false
Size:		1.09GB
Description:	pulled from docker.io/library/rust:1.89.0-slim@sha256:33219ca58c0dd38571fd3f87172b5bce2d9f3eb6f27e6e75efe12381836f71fa
Usage count:	11
Last used:	23 hours ago
Type:		regular

Any clues as to how to use --filter? From https://docs.docker.com/reference/cli/docker/buildx/du/ I only get "Provide filter values"...