BuildKit: Docker's Hidden Gem That Can Build Almost Anything

https://tuananh.net/2026/02/25/buildkit-docker-hidden-gem/

by jasonpeacock • 5 months ago

223 77 5 months ago

whalesalad2• 5 months ago

Folks, please fix your AI generated ascii artwork that is way out of alignment. This is becoming so prevalent - instant AI tell.

craftkiller3• 5 months ago

Are you on a phone? I loaded the article with both my phone and laptop. The ascii diagram was thoroughly distorted on my phone but it looked fine on my laptop.

whalesalad2• 5 months ago

Firefox on a 27" display. Could be the font being used to render.

unshavedyak• 5 months ago

I imagine it's not the AI then, but the site font/css/something. Seeing as it looks fine for me (Brave, Linux).

seneca• 5 months ago

I found it more jarring that they chose to use both Excalidraw and ascii art. What a strange choice.

tuananh4• 5 months ago

the hugo theme requires an image thumbnail. i just find one and use it :D

scuff3d2• 5 months ago

The "This is the key insight -" or "x is where it gets practical -", are dead give aways too. If I wanted an LLMs explanation of how it works, I can ask an LLM. When I see articles like this I'm expecting an actual human expert

slekker• 5 months ago

This one too: "It’s a proven pattern."

croes• 5 months ago

And waste time and energy again to get a similar result?

moochmooch2• 5 months ago

unfortunately, make is more well written software. I think ultimately Dockerfile was a failed iteration of Makefile. YAML & Dockerfile are poor interfaces for these types of applications.

The code first options are quite good these days, but you can get so far with make & other legacy tooling. Docker feels like a company looking to sell enterprise software first and foremost, not move the industry standard forward

great article tho!

craftkiller3• 5 months ago

Along similar lines, when I was reading the article I was thinking "this just sounds like a slightly worse version of nix". Nix has the whole content addressed build DAG with caching, the intermediate language, and the ability to produce arbitrary outputs, but it is functional (100% of the inputs must be accounted for in the hashes/lockfile, as opposed to Docker where you can run commands like `apk add firefox` which is pulling data from outside sources that can change from day to day, so two docker builds can end up with the same hash but different output, making it _not_ reproducible like the article falsely claims).

Edit: The claim about the hash being the same is incorrect, but an identical Dockerfile can produce different outputs on different machines/days whereas nix will always produce the same output for a given input.

jasonpeacock2• 5 months ago

You can network-jail your builds to prevent pulling from external repos and force the build environment to define/capture its inputs.

ricardobeat• 5 months ago

so two docker builds can end up with the same hash but different output

The cache key includes the state of the filesystem so I don’t think that would ever be true.

Regardless, the purpose of the tool is to generate [layer] images to be reused, exactly to avoid the pitfalls of reproducible builds, isn’t it? In the context of the article, what makes builds reproducible is the shared cache.

Izkata2• 5 months ago

whereas nix will always produce the same output for a given input.

If they didn't take shortcuts. I don't know if it's been fixed, but at one point Vuze in nix pulled in an arbitrary jar file from a URL. I had to dig through it because the jar had been updated at some point but not the nix config and it was failing at an odd place.

kccqzy2• 5 months ago

Make is timestamp based. That is a thoroughly out-of-date approach only suitable for a single computer. You want distributed hash-based caching in the modern world.

moochmooch2• 5 months ago

so use Bazel or buck2 if you need an iteration on make's handling of changed files. Bazel is much more serious of a project than buildkit. I'm not saying make is more functional that buildkit (it might be to some), I'm saying its better written software than buildkit. two separate things

stackskipton• 5 months ago

SRE here, I feel like both are just instructions how to get source code -> executable with docker/containers providing "deployable package" even if language does not compile into self-contained binary (Python, Ruby, JS, Java, .Net)

Also, there is nothing stopping you from creating a container that has make + tools required to compile your source code, writing a dockerfile that uses those tools to produce the output and leave it on the file system. Why that approach? Less friction for compiling since I find most make users have more pet build servers then cattle or making modifications can have a lot of friction due to conflicts.

bmitch3020• 5 months ago

I don't use buildkit for artifacts, but I do like to output images to an OCI Layout so that I can finish some local checks and updates before pushing the image to a registry.

But the real hidden power of buildkit is the ability to swap out the Dockerfile parser. If you want to see that in action, look at this Dockerfile (yes, that's yaml) used for one of their hardened images: https://github.com/docker-hardened-images/catalog/blob/main/...

tuananh4• 5 months ago

i did include a repo example on how to create custom frontend as well https://github.com/tuananh/apkbuild

kylegalbraith4• 5 months ago

I agree on both fronts! BuildKit frontends are not very well known but can be very powerful if you know how they work and how BuildKit transforms them.

verdverm6• 5 months ago

BuildKit also comes with a lot of pain. Dagger (a set of great interfaces to BuildKit in many languages) is working to remove it. Even their BuildKit maintainers think it's a good idea.

BuildKit is very cool tech, but painful to run at volume

Fun gotchya in BuildKit direct versus Dockerfiles, is the map iteration you loaded those ENV vars into consistent? No, that's why your cache keeps getting busted. You can't do this in the linear Dockerfile

kodama-lens• 5 months ago

I switched our entire container build setup to buildkit. No kaniko, no buildah, no dind. The great part is that you can split buildkitd and the buildctl.

Everything runs in its own docker runner. New buildkitd service for every job. Caching only via buildkit native cache export. Output format oci image compressed with zstd. Works pretty great so far, same or faster builds and we now create multi arch images. All on rootless runners by the way

verdverm6• 5 months ago

That's pretty cool, rootless would be nice, but more effort than we see in ROI currently. I'm using the Dagger SDK directly, no CLI or modules.

Had to recently make it so multiple versions can run on the same host, such that as developers change branches, which may be on different IaC'd versions (we launch on demand), we don't break LTS release branches.

Orygin• 5 months ago

That's my experience too. Coming from single threaded, single concurrent build Kaniko to parallel builds using Buildkit reduced build times by 2 to 3.

zaphirplane• 5 months ago

This is a strange double submission , the one with caps made it !

https://news.ycombinator.com/item?id=47152488

cyberax3• 5 months ago

Buildkit...

It sounds great in theory, but it JustDoesn'tWork(tm).

Its caching is plain broken, and the overhead of transmitting the entire build state to the remote computer every time is just busywork for most cases. I switched to Podman+buildah as a result, because it uses the previous dead simple Docker layered build system.

If you don't believe me, try to make caching work on Github with multi-stage images. Just have a base image and a couple of other images produced from it and try to use the GHA cache to minimize the amount of pulled data.

mid-kid• 5 months ago

How do you use buildah? with dockerfiles?

I find that buildah is sort of unbearably slow when using dockerfiles...

cyberax3• 5 months ago

It has a braindead cache checking, I've fixed it locally and I'm cleaning it up for the upstream submission. But otherwise, it's always faster for me than Buildkit.

hanikesn• 5 months ago

Why would you use the horrible GHA cache and not a much more efficient registry based cache?

cyberax3• 5 months ago

Registry cache...

It's yet one more incomprehensible Buildkit decision. The original Docker builder had a very simple cache system: it computed the layer hash and then checked the registry for its presence. Simple content-addressable caching.

Buildkit can NOT do this. Instead, it uses a single image as a dumping ground for the caches. If you have two builders using the same image, they'll step on each other's toes. GHA at least side-steps this.

But I tried the registry cache, and it didn't improve anything. So far, I was not able to get caching to work with multi-stage builds at all. There are open issues for that, dating back to 2020.

plantain• 5 months ago

Never figured out how to get the buildx cache to actually work reliably on ARM OS X. Horrible if you have to build x86 images regularly.

kylegalbraith4• 5 months ago

After building Depot [0] for the past three years, I can say I have a ton of scar tissue from running BuildKit to power our remote container builders for thousands of organizations.

It looks and sounds incredibly powerful on paper. But the reality is drastically different. It's a big glob of homegrown thoughts and ideas. Some of them are really slick, like build deduplication. Others are clever and hard to reason about, or in the worst case, terrifying to touch.

We had to fork BuildKit very early in our Depot journey. We've fixed a ton of things in it that we hit for our use case. Some of them we tried to upstream early on, but only for it to die on the vine for one reason or another.

Today, our container builders are our own version of BuildKit, so we maintain 100% compatibility with the ecosystem. But our implementation is greatly simplified. I hope someday we can open-source that implementation to give back and show what is possible with these ideas applied at scale.

[0] https://depot.dev/products/container-builds

skrtskrt• 5 months ago

It's a big glob of homegrown thoughts and ideas. Some of them are really slick, like build deduplication. Others are clever and hard to reason about, or in the worst case, terrifying to touch.

This is true of packaging and build systems in general. They are often the passion projects of one or a handful of people in an organization - by the time they have active outside development, those idiosyncratic concepts are already ossified.

It's really rare to see these sorts of projects decomposed into building blocks even just having code organization that helps a newcomer understand. Despite all the code being out in public, all the important reasoning about why certain things are the way they are is trapped inside a few dev's heads.

mikepurvis3• 5 months ago

As someone who has worked in the space for a while and been heavily exposed to nix, bazel, cmake, bake, and other systems, and also been in that "passion project" role, I think what I've found is that these kinds of systems are just plain hard to talk about. Even the common elements like DAGs cause most people's eyes to immediately glaze over.

Managers and executives are happy to hear that you made the builds faster or more reliable, so the infra people who care about this kind of thing don't waste time on design docs and instead focus on getting to a minimum prototype that demonstrates those improved metrics. Once you have that, then there's buy-in and the project is made official... but by then the bones have already been set in place, so design documentation ends up focused on the more visible stuff like user interface, storage formats, etc.

OTOH, bazel (as blaze) was a very intentionally designed second system at Google, and buildx/buildkit is similarly a rewrite of the container builder for Docker, so both of them should have been pretty free of accidental engineering in their early phases.

nl• 5 months ago

This is true of packaging and build systems in general. They are often the passion projects of one or a handful of people in an organization

This is a very insightful comment

mikepurvis3• 5 months ago

I introduced Depot at my org a few months ago and I've been very happy with it. Conceptually it's simple: a container builder that starts warm with all your previously built layers right there, same as it would be running local builds. But a lot goes into making it actually run smoothly, and the performance-focused breakdown that shows where steps depend on each other and how much time each is taking is great.

It's clear a ton of care has gone into the product, and I also appreciated you personally jumping onto some of my support tickets when I was just getting things off the ground.

kylegalbraith4• 5 months ago

Thank you for the very kind words and for your support. Depot is full of incredible people who love helping others. So while you might see me on a ticket from time to time, it’s really an entire team that is behind everything we do.

tuananh4• 5 months ago

Thanks for the insight Kyle. If Depot can open-source it, that would be amazing for the community.

kylegalbraith4• 5 months ago

Thank you for the post! It’s well done and you captured a lot of the concepts in BuildKit in an easy to understand way. Not an easy thing to do at all.

dewey• 5 months ago

Looks very interesting, am I reading it correctly that this is only available on AWS for "bring your own compute"?

Avamander• 5 months ago

Except anything that requires any non-trivial networking or hermetic building.

matheus-rr• 5 months ago

The --mount=type=cache for package managers is genuinely transformative once you figure it out. Before that, every pip install or apt-get in a Dockerfile was either slow (no caching) or fragile (COPY requirements.txt early and pray the layer cache holds).

What nobody tells you is that the cache mount is local to the builder daemon. If you're running builds on ephemeral CI instances, those caches are gone every build and you're back to square one. The registry cache backend exists to solve this but it adds enough complexity that most teams give up and just eat the slow builds.

The other underrated BuildKit feature is the ssh mount. Being able to forward your SSH agent into a build step without baking keys into layers is the kind of thing that should have been in Docker from day one. The number of production images I've seen with SSH keys accidentally left in intermediate layers is genuinely concerning.

candiddevmike• 5 months ago

I hate the nanny state behavior of docker build and not being allowed to modify files/data outside of the build container and cache, like having a NFS mount for sharing data in the build or copying files out of the build.

Let me have side effects, I'm a consenting adult and understand the consequences!!!

amluto3• 5 months ago

There is something wrong with the industry in which we think that, when a production build requires SSH keys, the problem is that the keys might leak into the build artifact.

stevenhuang3• 5 months ago

Keys leaking into the build artifact was never the concern.

It's about not having the private keys stored unknowingly in intermediate layers of a build container.

ltbarcly3• 5 months ago

But the problem is it's trash? I've repeatedly tried to do things documented to work that weren't implemented, or where the implementation is buggy.

Docker is profoundly bad software.

BuildKit: Docker's Hidden Gem That Can Build Almost Anything

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