docker: add docu references & remove duplicate code

docker.nix

@@ -147,23 +147,11 @@ let
     "${k}:x:${toString gid}:${lib.concatStringsSep "," members}";
   groupContents = (lib.concatStringsSep "\n" (lib.attrValues (lib.mapAttrs groupToGroup groups)));
 
-  defaultNixConf = {
-    sandbox = "false";
-    build-users-group = "nixbld";
-    trusted-public-keys = [ "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" ];
-  };
-
   nixConfContents =
-    (lib.concatStringsSep "\n" (
-      lib.mapAttrsToList (
-        n: v:
-        let
-          vStr = if builtins.isList v then lib.concatStringsSep " " v else v;
-        in
-        "${n} = ${vStr}"
-      ) (defaultNixConf // nixConf)
-    ))
-    + "\n";
+    pkgs.dockerTools.nixConf
+    {
+      build-users-group = "nixbld";
+    };
 
   userHome = if uid == 0 then "/root" else "/home/${uname}";
 
@@ -181,6 +169,8 @@ let
         name = "root-profile-env";
         paths = defaultPkgs;
       };
+      # doc/manual/source/command-ref/files/manifest.nix.md
+      # may get replaced by pkgs.buildEnv once manifest.json can get written
       manifest = pkgs.buildPackages.runCommand "manifest.nix" { } ''
         cat > $out <<EOF
         [
@@ -246,6 +236,7 @@ let
           set -x
           mkdir -p $out/etc
 
+          # may get replaced by pkgs.dockerTools.caCertificates
           mkdir -p $out/etc/ssl/certs
           ln -s /nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt $out/etc/ssl/certs
 
@@ -273,17 +264,21 @@ let
           mkdir -p $out${userHome}
           mkdir -p $out/nix/var/nix/profiles/per-user/${uname}
 
+          # 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
 
+          # see doc/manual/source/command-ref/files/channels.md
           ln -s ${channel} $out/nix/var/nix/profiles/per-user/${uname}/channels-1-link
           ln -s /nix/var/nix/profiles/per-user/${uname}/channels-1-link $out/nix/var/nix/profiles/per-user/${uname}/channels
 
+          # see doc/manual/source/command-ref/files/default-nix-expression.md
           mkdir -p $out${userHome}/.nix-defexpr
           ln -s /nix/var/nix/profiles/per-user/${uname}/channels $out${userHome}/.nix-defexpr/channels
           echo "${channelURL} ${channelName}" > $out${userHome}/.nix-channels
 
+          # may get replaced by pkgs.dockerTools.binSh & pkgs.dockerTools.usrBinEnv
           mkdir -p $out/bin $out/usr/bin
           ln -s ${pkgs.coreutils}/bin/env $out/usr/bin/env
           ln -s ${pkgs.bashInteractive}/bin/bash $out/bin/sh