nixos/nscd: run with a dynamic user

nscd doesn't create any files outside of /run/nscd unless the nscd.conf
"persistent" option is used, which we don't do by default. Therefore it
doesn't matter what UID/GID we run this service as, so long as it isn't
shared with any other running processes.

/run/nscd does need to be owned by the same UID that the service is
running as, but systemd takes care of that for us thanks to the
RuntimeDirectory directive.

If someone wants to turn on the "persistent" option, they need to
manually configure users.users.nscd and systemd.tmpfiles.rules so that
/var/db/nscd is owned by the same user that nscd runs as.

In an all-defaults boot.isContainer configuration of NixOS, this removes
the only user which did not have a pre-assigned UID.

nixos/modules/services/system/nscd.conf

@@ -6,7 +6,6 @@
 # fallback to trying to handle the request by itself. Which won't work as glibc
 # is not aware of the path in which the nss modules live.  As a workaround, we
 # have `enable-cache yes` with an explicit ttl of 0
-server-user             nscd
 threads                 1
 paranoia                no
 debug-level             0

nixos/modules/services/system/nscd.nix

@@ -39,11 +39,6 @@ in
   config = mkIf cfg.enable {
     environment.etc."nscd.conf".text = cfg.config;
 
-    users.users.nscd =
-      { isSystemUser = true;
-        description = "Name service cache daemon user";
-      };
-
     systemd.services.nscd =
       { description = "Name Service Cache Daemon";
 
@@ -60,6 +55,8 @@ in
         serviceConfig =
           { ExecStart = "@${pkgs.glibc.bin}/sbin/nscd nscd";
             Type = "forking";
+            User = "nscd";
+            DynamicUser = true;
             RuntimeDirectory = "nscd";
             PIDFile = "/run/nscd/nscd.pid";
             Restart = "always";