add instructions to run under systemd user services

scripts/systemd.md

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+# Forgejo Runner with systemd User Services
+
+It is possible to use systemd's user services together with 
+[podman](https://podman.io/) to run `forgejo-runner` using a normal user 
+account without any privileges and automatically start on boot.
+
+This was last tested on Fedora 39 on 2024-02-19, but should work elsewhere as 
+well.
+
+Place the `forgejo-runner` binary in `/usr/local/bin/forgejo-runner` and make
+sure it can be executed (`chmod +x /usr/local/bin/forgejo-runner`).
+
+Install and enable `podman` as a user service:
+
+```bash
+$ sudo dnf -y install podman
+```
+
+You *may* need to reboot your system after installing `podman` as it 
+modifies some system configuration(s) that may need to be activated. Without
+rebooting the system my runner errored out when trying to set firewall rules, a
+reboot fixed it.
+
+Enable `podman` as a user service:
+
+```
+$ systemctl --user start podman.socket
+$ systemctl --user enable podman.socket
+```
+
+Make sure processes remain after your user account logs out:
+
+```bash
+$ loginctl enable-linger
+```
+
+Create the file `/etc/systemd/user/forgejo-runner.service` with the following
+content:
+
+```
+[Unit]
+Description=Forgejo Runner
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/forgejo-runner daemon
+Restart=on-failure
+
+[Install]
+WantedBy=default.target
+```
+
+Now activate it as a user service:
+
+```bash
+$ systemctl --user daemon-reload
+$ systemctl --user start forgejo-runner
+$ systemctl --user enable forgejo-runner
+```
+
+To see/follow the log of `forgejo-runner`:
+
+```bash
+$ journalctl -f -t forgejo-runner
+```
+
+If you reboot your system, all should come back automatically.