[clhodapp]

Docker can resolve dependencies in a very similar manner to nix, via multi-stage builds. Each FROM makes one dependency available. However, you can only have direct access to the content from one of the dependencies resolved this way. The other ones, you have to COPY over the relevant content --from at build time.

[cpuguy83]

I'm not exactly sure what you are referring to here.

You can have as many "FROM"'s as you want. "FROM scratch" along with "ADD" is also valid (for non-image dependencies).

From there you do not need to copy things, you can mount the reference into another stage directly.

Also, this is the Dockerfile format, the underlying build API's are _far_ more powerful than what Dockerfile exposes.

[clhodapp]

You're totally right about the underlying container image format being much more powerful than what you can leverage from a Dockerfile. That's exactly the thing that makes nix a better Docker image builder than Docker! It leverages that power to create images that properly use layers to pull in many dependencies at the same time, and in a way that they can be freely shared in a composable way across multiple different images!

A Docker FROM is essentially the equivalent of a dependency in nix... but each RUN only has access to the stuff that comes from the FROM directly above it plus content that has been COPY-ed across (and COPY-ing destroys the ability to share data with the source of the COPY). For Docker to have a similar power to nix at building Docker images, you would need to be be able to union together an arbitrary number of FROM sources to create a composed filesystem.

[cpuguy83]

Even with the Dockerfile format you can union those filesystems (COPY --link).

People use the Dockerfile format because it is accessible. You can still use "docker build" with whatever format you want, or drive it completely via API where you have the full power of the system.

[clhodapp]

I actually hadn't heard of COPY --link, but it's interesting because it seems to finally create a way of establishing a graph of dependencies from a Dockerfile! It doesn't sound like it's quite good enough to let you build a nix-like system, though, because it can only copy to empty directories (at least based on what the docs say). You really need the ability to e.g. union together a bunch of libraries to from a composite /lib.

I'm not sure what you mean by 'You can still use "docker build" with whatever format you want'. As far as I'm aware, "docker build" can only build Dockerfiles.

I'm also not sure what you mean when you mention gaining extra abilities to make layered images via the API. As far as I can tell, the only way to make images from the API is to either run Dockerfiles or to freeze a running container's filesystem into an image.

[cpuguy83]

docker build is backed by buildkit, which is available as a grpc service ("docker build" is a grpc client/server).

Buildkit operates on "LLB", which would be equivalent to llvm IR. Dockerfile is a frontend. Buildkit has the Dockerfile frontend built in, but you can use your own frontend as well.

If you ever see "syntax=docker/dockerfile:1.6", as an example, this triggers buildkit to fire up a container with that image and uses that as the front end instead of the builtin Dockerfile frontend. Docker doesn't actually care what the format is.

Alternatively, you can access the same frontend api's from a client (which, technically, a frontend is just a client).

Frontends generate LLB which gets sent to the solver to execute.

[clhodapp]

OK, wow, this is interesting indeed. I didn't realize just how much of a re-do of the build engine Buildkit was, I had just thought of it as a next-gen internal build engine, running off of Dockerfiles.

Applying this information to the topic at hand:

Given what Buildkit actually does, I bet someone could create a compiler that does a decent job transforming nix "derivations", the underlying declarative format that the nix daemon uses to run builds, into these declarative Buildkit protobuf objects and run nix builds on Buildkit instead of the nix daemon. To make this concrete, we would be converting from something that looked like this: https://gist.github.com/clhodapp/5d378e452d1c4993a5e35cd043d.... So basically, run "bash" with those args and environment variables, with those derivations show below already built and their outputs made visible.

Once that exists, it should also be possible to create a frontend that consumes a list of nix "installables" (how you refer to specific concrete packages) and produces an oci image out of the nix package repository, without relying on the nix builder to actually run any of it.

This would subsume the purpose of e.g. https://nixery.dev/