man-db cache generation fails when building inside Nix Docker image

[pkoenig10]

Describe the bug

I'm attempting to use the official Nix Docker image to build a NixOS LiveCD on a system without Nix installed.

I have the following iso.nix file:

{
  ...
}:

{
  imports = [
    <nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix>
  ];

  programs.fish.enable = true;
}

And I'm attempting to build the LiveCD it with the following command:

docker run --rm \
    -v ./iso.nix:/iso.nix \
    nixos/nix:2.31.2 \
    nix-build '<nixpkgs/nixos>' -A config.system.build.isoImage -I nixos-config=/iso.nix

But this fails with the following error:

building '/nix/store/rrzpw22bv3q10map05gbp0s5mgffr9ic-man-cache.drv'...
error: Cannot build '/nix/store/rrzpw22bv3q10map05gbp0s5mgffr9ic-man-cache.drv'.
       Reason: builder failed with exit code 2.
       Output paths:
         /nix/store/iryvzki6r36ds2iq64pyrs32lsb5qiwz-man-cache

This happens because src/man_db.conf.in contains the following line:

MANDB_MAP /nix/var/nix/profiles/default/share/man /var/cache/man/nixpkgs

When building this ISO in NixOS, there is no such file:

$ ls -la /nix/var/nix/profiles/
/nix/var/nix/profiles/:
total 0
drwxr-xr-x 3 root root 100 Oct  3 02:11 .
drwxr-xr-x 7 root root 160 Oct  3 02:11 ..
drwxr-xr-x 3 root root  60 Oct  3 02:11 per-user
lrwxrwxrwx 1 root root  13 Oct  3 02:11 system -> system-1-link
lrwxrwxrwx 1 root root  88 Oct  3 02:11 system-1-link -> /nix/store/zskwzx88124z5f56zdidhp8y34wn23kd-nixos-system-nixos-24.11.713387.107d5ef05c0b

But when building this ISO in the Nix Docker image, that file exists:

/nix/var/nix/profiles:
total 8
drwxr-xr-x 1 root root 58 Jan  1  1980 .
drwxr-xr-x 1 root root 64 Oct  3 02:12 ..
lrwxrwxrwx 1 root root 36 Jan  1  1980 default -> /nix/var/nix/profiles/default-1-link
lrwxrwxrwx 1 root root 60 Jan  1  1980 default-1-link -> /nix/store/0djq5pgvmzhl7v132slkpqfm9nan0i13-root-profile-env
drwxr-xr-x 1 root root  8 Jan  1  1980 per-user

It seems that this is due to the configuration here:

nix/docker.nix

Lines 310 to 313 in fdea162

	# see doc/manual/source/command-ref/files/profiles.md
	ln -s ${profile} $out/nix/var/nix/profiles/default-1-link
	ln -s /nix/var/nix/profiles/default-1-link $out/nix/var/nix/profiles/default
	ln -s /nix/var/nix/profiles/default $out${userHome}/.nix-profile

I have relatively little experience with Nix, so it's certainly possible I'm doing something wrong.

Steps To Reproduce

1. Create the following iso.nix file

   {
   ...
   }:
   
   {
     imports = [
       <nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix>
     ];
   
   
     documentation.man.generateCaches = true;
   }

2. Run the following command

   docker run --rm \
       -v ./iso.nix:/iso.nix \
       nixos/nix:2.31.2 \
       nix-build '<nixpkgs/nixos>' -A config.system.build.isoImage -I nixos-config=/iso.nix
   

Expected behavior

I expected the build to complete successfully and produce an ISO image.

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates