contrib/systemd/labgrid-exporter, doc: document DynamicUser and helpers

The helpers with sudo on exporters cannot be used together with
systemd's DynamicUser. This directive enforces the NoNewPrivileges
directive, which leads to the following error:

sudo: The "no new privileges" flag is set, which prevents sudo from running as root.
sudo: If sudo is running in a container, you may need to adjust the container configuration to disable the flag.

Since DynamicUser introduced in 0a2deef
("contrib/systemd/labgrid-exporter: improve service") is a useful
feature, switch to User and Group only when using helpers.

Signed-off-by: Tomas Novotny <tomas@novotny.cz>

Author: stoupa-cz

contrib/systemd/labgrid-exporter.service

@@ -11,6 +11,7 @@ EnvironmentFile=-/etc/environment
 ExecStart=/path/to/labgrid/venv/bin/labgrid-exporter /etc/labgrid/exporter.yaml
 Restart=always
 RestartSec=30
+# Replace 'DynamicUser=yes' with 'User=labgrid' and 'Group=labgrid' when using labgrid helpers with sudo.
 DynamicUser=yes
 # Adjust to your distribution (most often "dialout" or "tty")
 SupplementaryGroups=dialout plugdev

doc/getting_started.rst

@@ -379,6 +379,15 @@ Follow these instructions to install the systemd files on your machine(s):
    installation paths of your distribution.
 #. Adapt the ``ExecStart`` paths of the service files to the respective Python
    virtual environments of the coordinator and exporter.
+#. If you use labgrid helpers (see :file:`helpers/`) with sudo on exporter,
+   replace ``DynamicUser`` with ``User`` and ``Group`` in the
+   :file:`labgrid-exporter.service`:
+
+   .. code-block::
+
+      User=labgrid
+      Group=labgrid
+
 #. Adjust the ``SupplementaryGroups`` option in the
    :file:`labgrid-exporter.service` file to your distribution so that the
    exporter gains read and write access on TTY devices (for ``ser2net``); most