docker: add docu references & remove duplicate code

docker.nix

@@ -176,24 +176,20 @@ let
     "${k}:x:${toString gid}:${lib.concatStringsSep "," members}";
   groupContents = (lib.concatStringsSep "\n" (lib.attrValues (lib.mapAttrs groupToGroup groups)));
 
-  defaultNixConf = {
-    sandbox = "false";
+  toConf =
+    with pkgs.lib.generators;
+    toKeyValue {
+      mkKeyValue = mkKeyValueDefault {
+        mkValueString = v: if lib.isList v then lib.concatStringsSep " " v else mkValueStringDefault { } v;
+      } " = ";
+    };
+
+  nixConfContents = toConf {
+    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} = ${vStr}"
-      ) (defaultNixConf // nixConf)
-    ))
-    + "\n";
-
   userHome = if uid == 0 then "/root" else "/home/${uname}";
 
   baseSystem =
@@ -211,10 +207,7 @@ let
           echo "[]" > $out/manifest.nix
         fi
       '';
-      rootEnv = buildPackages.buildEnv {
-        name = "root-profile-env";
-        paths = defaultPkgs;
-      };
+      # doc/manual/source/command-ref/files/manifest.nix.md
       manifest = buildPackages.runCommand "manifest.nix" { } ''
         cat > $out <<EOF
         [
@@ -244,11 +237,15 @@ let
         ]
         EOF
       '';
-      profile = buildPackages.runCommand "user-environment" { } ''
-        mkdir $out
-        cp -a ${rootEnv}/* $out/
-        ln -s ${manifest} $out/manifest.nix
-      '';
+      profile = buildPackages.buildEnv {
+        name = "root-profile-env";
+        paths = defaultPkgs;
+
+        postBuild = ''
+          mv $out/manifest $out/manifest.nix
+        '';
+        inherit manifest;
+      };
       flake-registry-path =
         if (flake-registry == null) then
           null
@@ -280,6 +277,7 @@ 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
 
@@ -307,16 +305,20 @@ 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
 
+          # 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 ${lib.getExe' coreutils-full "env"} $out/usr/bin/env
           ln -s ${lib.getExe bashInteractive} $out/bin/sh