One alternative is to put noDumpEnvVars=1 before the sourcing of $stdenv/setup. Not sure that is appropriate though, since it is building and just happen to wrap the build inside proot.

Thank you very much for your efforts, @terlar.
I can confirm that this patch solves the build regressions I described on the PR to master.

I'm curious to know how you made out the PR/commit that introduced the regression. I tried throwing git bisect at the problem, but I didn't get to the source.

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 429601 --package dockerTools.tests.docker-tools
Commit: 6ccf14ba235944f96f67e252812d59ef1fb15e38

x86_64-linux

@juliusrickert The procedure was quite painful as it was changes in the generic-builder. I explained it here:
#417132 (comment)

I had to drill down to the code (customisationLayer) that was throwing the error, as otherwise I had to rebuild 1000s of packages.

builtins.elemAt (pkgs.dockerTools.streamLayeredImage {
  name = "debug-stream-layered-image";
  enableFakechroot = true;
  fakeRootCommands = ''
    echo Is it working?
  '';
}).conf.layersJsonFile.exclude_paths 1

Even with this I had to rebuild a few 100 packages, so it took a few hours to bisect.