docker: Allow building for non-root user

Add options uid, gid, uname, and gname to docker.nix.

Setting these to e.g. 1000, 1000, "user", "user" will build an image
which runs and allows using Nix as that user.

docker.nix

@@ -9,6 +9,10 @@
 , maxLayers ? 100
 , nixConf ? {}
 , flake-registry ? null
+, uid ? 0
+, gid ? 0
+, uname ? "root"
+, gname ? "root"
 }:
 let
   defaultPkgs = with pkgs; [
@@ -50,6 +54,15 @@ let
       description = "Unprivileged account (don't use!)";
     };
 
+  } // lib.optionalAttrs (uid != 0) {
+    "${uname}" = {
+      uid = uid;
+      shell = "${pkgs.bashInteractive}/bin/bash";
+      home = "/home/${uname}";
+      gid = gid;
+      groups = [ "${gname}" ];
+      description = "Nix user";
+    };
   } // lib.listToAttrs (
     map
       (
@@ -70,6 +83,8 @@ let
     root.gid = 0;
     nixbld.gid = 30000;
     nobody.gid = 65534;
+  } // lib.optionalAttrs (gid != 0) {
+    "${gname}".gid = gid;
   };
 
   userToPasswd = (
@@ -150,6 +165,8 @@ let
     in
       "${n} = ${vStr}") (defaultNixConf // nixConf))) + "\n";
 
+  userHome = if uid == 0 then "/root" else "/home/${uname}";
+
   baseSystem =
     let
       nixpkgs = pkgs.path;
@@ -237,26 +254,26 @@ let
       mkdir -p $out/etc/nix
       cat $nixConfContentsPath > $out/etc/nix/nix.conf
 
-      mkdir -p $out/root
+      mkdir -p $out${userHome}
-      mkdir -p $out/nix/var/nix/profiles/per-user/root
+      mkdir -p $out/nix/var/nix/profiles/per-user/${uname}
 
       ln -s ${profile} $out/nix/var/nix/profiles/default-1-link
       ln -s $out/nix/var/nix/profiles/default-1-link $out/nix/var/nix/profiles/default
-      ln -s /nix/var/nix/profiles/default $out/root/.nix-profile
+      ln -s /nix/var/nix/profiles/default $out${userHome}/.nix-profile
 
-      ln -s ${channel} $out/nix/var/nix/profiles/per-user/root/channels-1-link
+      ln -s ${channel} $out/nix/var/nix/profiles/per-user/${uname}/channels-1-link
-      ln -s $out/nix/var/nix/profiles/per-user/root/channels-1-link $out/nix/var/nix/profiles/per-user/root/channels
+      ln -s $out/nix/var/nix/profiles/per-user/${uname}/channels-1-link $out/nix/var/nix/profiles/per-user/${uname}/channels
 
-      mkdir -p $out/root/.nix-defexpr
+      mkdir -p $out${userHome}/.nix-defexpr
-      ln -s $out/nix/var/nix/profiles/per-user/root/channels $out/root/.nix-defexpr/channels
+      ln -s $out/nix/var/nix/profiles/per-user/${uname}/channels $out${userHome}/.nix-defexpr/channels
-      echo "${channelURL} ${channelName}" > $out/root/.nix-channels
+      echo "${channelURL} ${channelName}" > $out${userHome}/.nix-channels
 
       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
 
     '' + (lib.optionalString (flake-registry-path != null) ''
-      nixCacheDir="/root/.cache/nix"
+      nixCacheDir="${userHome}/.cache/nix"
       mkdir -p $out$nixCacheDir
       globalFlakeRegistryPath="$nixCacheDir/flake-registry.json"
       ln -s ${flake-registry-path} $out$globalFlakeRegistryPath
@@ -268,7 +285,7 @@ let
 in
 pkgs.dockerTools.buildLayeredImageWithNixDb {
 
-  inherit name tag maxLayers;
+  inherit name tag maxLayers uid gid uname gname;
 
   contents = [ baseSystem ];
 
@@ -279,25 +296,28 @@ pkgs.dockerTools.buildLayeredImageWithNixDb {
   fakeRootCommands = ''
     chmod 1777 tmp
     chmod 1777 var/tmp
+    chown -R ${toString uid}:${toString gid} .${userHome}
+    chown -R ${toString uid}:${toString gid} nix
   '';
 
   config = {
-    Cmd = [ "/root/.nix-profile/bin/bash" ];
+    Cmd = [ "${userHome}/.nix-profile/bin/bash" ];
+    User = "${toString uid}:${toString gid}";
     Env = [
-      "USER=root"
+      "USER=${uname}"
       "PATH=${lib.concatStringsSep ":" [
-        "/root/.nix-profile/bin"
+        "${userHome}/.nix-profile/bin"
         "/nix/var/nix/profiles/default/bin"
         "/nix/var/nix/profiles/default/sbin"
       ]}"
       "MANPATH=${lib.concatStringsSep ":" [
-        "/root/.nix-profile/share/man"
+        "${userHome}/.nix-profile/share/man"
         "/nix/var/nix/profiles/default/share/man"
       ]}"
       "SSL_CERT_FILE=/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"
       "GIT_SSL_CAINFO=/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"
       "NIX_SSL_CERT_FILE=/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"
-      "NIX_PATH=/nix/var/nix/profiles/per-user/root/channels:/root/.nix-defexpr/channels"
+      "NIX_PATH=/nix/var/nix/profiles/per-user/${uname}/channels:${userHome}/.nix-defexpr/channels"
     ];
   };