Create a derivation in Nix to only copy some files

I have some files in a folder that I want to create a derivation out of to pass them as inputs to dockerTools.buildImage later.

I'm not sure how to do this. This doesn't work:

database = pkgs.stdenv.mkDerivation {
  name = "database";
  src = "./";
  phases = [ "installPhase" ];
  installPhase = ''
      pwd
      ls -al
      echo $src
      cp $src/.env $out
      cp $src/20221002.sqlite $out
  '';
};

Because cp can't find the files.

Turns out this is what's necessary for this to work:

database = pkgs.stdenv.mkDerivation {
  name = "database";
  src = ./.;
  phases = [ "unpackPhase" "installPhase" ];
  installPhase = ''
    mkdir -p $out
    cp $src/.env $out
    cp $src/20221002.sqlite $out
  '';
};

The src attribute needs to point to the current directory and then everything in there will be copied over.

The copying only happens when unpackPhase is part of the phases.

Then in the installPhase we need to drop into shell to do the actual work. $out means the output path of the derivation and $src is set to the folder of the src. In many cases $out is just a file but in this case since we want it to contain two files, we have to mkdir it first and then we can cp both files into it.