Fix "Migrating to a new server" flow due to dynamic user/group creation

docs/maintenance-migrating.md

@@ -1,8 +1,10 @@
 # Migrating to new server
 
 1. Prepare by lowering DNS TTL for your domains (`matrix.DOMAIN`, etc.), so that DNS record changes (step 4 below) would happen faster, leading ot less downtime
-2. Stop all services on the old server and make sure they won't be starting again. Execute this on the old server: `systemctl disable --now matrix*` 
+2. Stop all services on the old server and make sure they won't be starting again. Execute this on the old server: `systemctl disable --now matrix*`
 3. Copy directory `/matrix` from the old server to the new server. Make sure to preserve ownership and permissions (use `cp -p` or `rsync -ar`)!
 4. Make sure your DNS records are adjusted to point to the new server's IP address
 5. Remove old server from the `inventory/hosts` file and add new server.
-6. Run `ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start`. This will create the matrix user and group and start all services on the new server
+6. Run `ansible-playbook -i inventory/hosts setup.yml --tags=setup-system-user`. This will create the `matrix` user and group on the new server
+7. Because the `matrix` user and group are created dynamically on each server, the user/group id may differ between the old and new server. We suggest that you adjust ownership of `/matrix` files manually by running this on the new server: `chown -R matrix:matrix /matrix`.
+8. Run `ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start` to finish the installation and start all services

roles/matrix-base/tasks/main.yml

@@ -18,6 +18,7 @@
   when: run_setup|bool
   tags:
     - always
+    - setup-system-user
 
 - import_tasks: "{{ role_path }}/tasks/setup_matrix_base.yml"
   when: run_setup|bool