Nixpacks Feature Request，Support setting nix image source address

[mutou981]

Feature request

In China, accessing nix addresses is very slow and requires a mirror address access

...
#8 3372.3 copying path '/nix/store/h8d2j0prdf7pnpgyrgkxrrbfwnmxbq6y-binutils-wrapper-2.40' from 'https://cache.nixos.org'...
#8 4545.9 copying path '/nix/store/yfaywdajl7wr612khypmds6rd69pgpdw-icu4c-73.2-dev' from 'https://cache.nixos.org'...
#8 4699.9 copying path '/nix/store/5l687mklyr9rhhbvvpvi93zv0zbbi4vg-nodejs-18.18.2' from 'https://cache.nixos.org'...
#8 8860.1 copying path '/nix/store/zlzz2z48s7ry0hkl55xiqp5a73b4mzrg-gcc-wrapper-12.3.0' from 'https://cache.nixos.org'...
#8 8860.7 copying path '/nix/store/bbxdw4rgwwl3gnajri82yidr1nlsfskf-stdenv-linux' from 'https://cache.nixos.org'...
...

These copies took 1 hour to execute

I also want to support image source address settings for package management tools such as npm

Motivation

No response

Contribution

No response

[mutou981]

{setup_copy_cmds}
RUN nix --help
RUN ls /etc/nix
RUN echo 'substituters = https://mirrors.cernet.edu.cn/nix-channels/store https://cache.nixos.org/' > /etc/nix/nix.conf
RUN nix-channel --add https://mirrors.cernet.edu.cn/nix-channels/nixos-22.11 nixos && nix-channel --update
ENV NIX_PATH nixpkgs=https://mirrors.cernet.edu.cn/nix-channels/nixos-22.11/nixexprs.tar.xz
{nix_install_cmds}
{apt_pkgs_str}
{assets_copy_cmd}
...

I tried setting it this way, but it didn't work?

 => [stage-0  1/12] FROM ghcr.io/railwayapp/nixpacks:ubuntu-1716249803@sha256:5ef4d2021c418fcaabb56cea5ea9e24a5fdedfe27f58062d968  0.0s
 => [internal] load build context                                                                                                  0.0s
 => => transferring context: 2.61kB                                                                                                0.0s
 => CACHED [stage-0  2/12] WORKDIR /app/                                                                                           0.0s
 => CACHED [stage-0  3/12] RUN nix --help                                                                                          0.0s
 => CACHED [stage-0  4/12] RUN ls /etc/nix                                                                                         0.0s
 => CACHED [stage-0  5/12] RUN echo 'substituters = https://mirrors.cernet.edu.cn/nix-channels/store https://cache.nixos.org/' >   0.0s
 => CACHED [stage-0  6/12] RUN nix-channel --add https://mirrors.cernet.edu.cn/nix-channels/nixos-22.11 nixos && nix-channel --up  0.0s
 => CACHED [stage-0  7/12] COPY .nixpacks/nixpkgs-bf446f08bff6814b569265bef8374cfdd3d8f0e0.nix .nixpacks/nixpkgs-bf446f08bff6814b

 > [stage-0  8/12] RUN nix-env -if .nixpacks/nixpkgs-bf446f08bff6814b569265bef8374cfdd3d8f0e0.nix && nix-collect-garbage -d:
16.76 warning: error: unable to download 'https://github.com/NixOS/nixpkgs/archive/bf446f08bff6814b569265bef8374cfdd3d8f0e0.tar.gz': HTTP error 302 (curl error: SSL connect error); retrying in 304 ms

Still from github pull

[orkunzozturk]

i face the same problem. download speeds from china are extremely slow