Remote Builds

After reading Using Nix with Dockerfiles I wanted to understand how to use dockerTools.buildImage to build a Docker image, instead of relying on a Dockerfile.

The issue: I'm on a M1 MacBook, an aarch64 machine. The Docker on this machine runs within an aarch64 Linux VM. Naively building a nix flake means it will build for aarch64 macOS and that then cannot run within the Docker container. So I needed to understand how to either cross-compile or use a remote builder.

I went for the latter, using my x86_64-linux server with nix installed as a remote builder.

I started with the test command from the Remote Builds docs, slightly modified:

nix build --impure --expr '(with import <nixpkgs> { system = "x86_64-linux"; }; runCommand "foo" {} "uname > $out")' --builders 'ssh://builder x86_64-linux' --max-jobs 0 -vvv

--max-jobs 0 will ensure it won't run any local tasks and -vvv will show all the debug output.

This starts downloading nixpkgs and stuff and then ... fail:

error: unable to start any build; either increase '--max-jobs' or enable remote builds.

Unhelpful. The -vvv was necessary to even get any understanding of what's failing. Close to the top one can see this:

ignoring the client-specified setting 'builders', because it is a restricted setting and you are not a trusted user

The docs about trusted-users say that adding users there essentially give that user root rights. So let's not do that and instead configure the builder machine in /etc/nix/machines:

ssh://builder x86_64-linux

I also needed to set the user and the SSH key:


Apparently builders = @/etc/nix/machines is the default, but if not you can set that in /etc/nix/nix.conf. After that a restart of the nix daemon will be necessary:

sudo launchctl kickstart -k system/org.nixos.nix-daemon

Re-running the nix build --impure ... will fail again:

error: unexpected end-of-file
error: builder for '/nix/store/6ji85w7v51fs3x21szvbgmx4dj0vpjqs-foo.drv' failed with exit code 1;
       last 10 log lines:
       > error: you are not privileged to build input-addressed derivations
       > debug1: Exit status 1
       For full logs, run 'nix log /nix/store/6ji85w7v51fs3x21szvbgmx4dj0vpjqs-foo.drv'.

Sounds very similar to the initial issue. This time I set trusted-users = jer in /etc/nix/nix.conf on the builder machine. Then restarted the nix daemon with:

systemctl restart nix-daemon

Now the nix build on macOS succeeds and:

$ cat result

Building Docker images for x86_64

Last but not least I can build the Docker image for x86_64 now. The full example is in github:badboy/flask-nix-example.

nix build '.#packages.x86_64-linux.dockerImage' --max-jobs 0

Then load it:

docker load < result

And finally run the container:

docker run -it --rm -p 5001:5000 --platform linux/amd64 flask-example