Add systemd service unit example

[mk-fg]

Haven't found systemd unit next to Dockerfile and thought that maybe one will be useful in the repository.

Not sure if maybe this is deliberately left up to distros, but given that releases supply simple binaries, maybe useful to have for people who use those in some systemd-enabled env (most linux distros, incl. in VMs or nspawn/lxc containers).

This unit sets up everything-readonly-but-one-homedir container (DynamicUser=yes implies most of it).
With static Go binary, it might also be possible to configure something simplier like TemporaryFileSystem=/ + BindReadOnlyPaths='/proc /usr/bin/matterircd:/matterircd' to basically get same thing as Docker container in fs namespace, but dunno if daemon might still need libc and its stuff in /etc with some builds.

Since extremely verbose and potentially sensitive slack logging is always set at "debug" level, example includes an ExecStart= which would dump it to logfiles instead of sharing it into system logs (bad idea for such noisy/sensitive stuff).

[42wim]

Looks good as a starting point that people can modify, thanks!