[yjftsjthsd-h]

Right; you could write a Dockerfile that went something like

    FROM base-image@e70197813aa3b7c86586e6ecbbf0e18d2643dfc8a788aac79e8c906b9e2b0785
    RUN pkg install foo=1.2.3 bar=2.3.4
    RUN git clone https://some/source.git && cd source && git checkout f8b02f5809843d97553a1df02997a5896ba3c1c6
    RUN gcc --reproducible-flags source/foo.c -o foo

but that's (IME) really rare; you're more likely to find `FROM debian:10` (which isn't too likely to change but is not pinned) and `RUN git clone -b v1.2.3 repo.git` (which is probably fixed but could change)...

And then there's the Dockerfiles that just `RUN git clone repo.git` and run with whatever happened to be in the latest commit at the moment...

[janjongboom]

And that assumes that `foo` and `bar` are not overwritten or deleted in your package repository, and that the git repository remains available.

[nullify88]

Maintaining something like that is a pain unless you have tooling like Renovate to inform and update the digests and versions.

[cpuguy83]

It is likely just as rare for someone to use nix for this, though.

[yjftsjthsd-h]

Possible; I don't have a feel for the relative likelihoods. I think the thing nix has going for it is that you can write a nix package definition without having to actually hardcode anything in and nix itself will give you the defaults to make ex. compilers be deterministic/reproducible, and automate handling flake.lock so you don't have to actually pay attention to the pins yourself. Or put differently; you can make either one reproducible, but nix is designed to help you do that while docker really doesn't care.

[clhodapp]

It's actually how nix works by default. When you pull in a dependency, you are actually pulling in a full description of how to build it. And it pulls in full descriptions of how to build its dependencies and so on.

The only reason nix isn't dog slow is that it has really strong caching so it doesn't have to build everything from source.

[cpuguy83]

This is literally how docker works as well. The difference is docker doesn't bring a toolchain for those artifacts.

[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.

[MadnessASAP]

If you're using Nix, that is what you are ultimately producing, it's buried under significant amounts of boilerplate and sensible defaults. Ultimately the output of Nix (called a derivation) reads a lot like a pile of references, build instructions, and checksums.

[isbvhodnvemrwvn]

I think their point was that number of people who use nix is a rounding error, perhaps due to poor user experience.