fix(docker): declaratively apply groupmod --non-unique for IPA GID conflict

NixOS's group activation uses plain groupmod, which silently skips the
GID change when SSSD exposes the same GID (50010) via the IPA
docker-access group through NSS.  Add an activation script that runs
after the normal 'groups' step and applies groupmod --non-unique so the
local docker group can share GID 50010 with the SSSD-provided IPA group.
If the GID actually changes on a live system the script also restarts
docker.socket + docker.service so the socket is recreated with the new
GID.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/docker/enable-service.nix

@@ -1,5 +1,8 @@
 { lib, pkgs, vars, ... }:
 
+let
+  gid = toString vars.dockerAccessGid;
+in
 {
   virtualisation.docker = {
     enable = true;
@@ -16,4 +19,27 @@
     docker-compose
     docker-buildx
   ];
+
+  # NixOS's group activation uses plain `groupmod` without --non-unique.
+  # When SSSD is active it exposes the IPA "docker-access" group at
+  # vars.dockerAccessGid via NSS, so groupmod sees that GID as already in
+  # use and silently skips the change (warning: "not applying GID change").
+  # This script runs after the normal "groups" step and applies the change
+  # with --non-unique (which lets the local docker group share the GID with
+  # the SSSD-provided IPA group). If the GID actually changed it also
+  # restarts docker.socket so the socket is recreated with the new GID.
+  system.activationScripts.docker-group-gid = {
+    deps = [ "groups" ];
+    text = ''
+      current=$(grep "^docker:" /etc/group | cut -d: -f3)
+      if [ "$current" != "${gid}" ]; then
+        ${pkgs.shadow}/bin/groupmod --non-unique -g ${gid} docker
+        if ${pkgs.systemd}/bin/systemctl is-active --quiet docker.socket; then
+          ${pkgs.systemd}/bin/systemctl stop docker.service docker.socket
+          rm -f /var/run/docker.sock
+          ${pkgs.systemd}/bin/systemctl start docker.socket docker.service
+        fi
+      fi
+    '';
+  };
 }