add WIP draft-example.nsenter (an systemd system service example configured with User=)

[eriksjolund]

Add systemd system service example (configured with User=)

Currently work-in-progress

The implementation makes use of

ExecStart=bash -c "exec nsenter \
    --preserve-credentials \
    --net=/proc/$(cat $XDG_RUNTIME_DIR/containers/networks/aardvark-dns/aardvark.pid)/ns/net \
    --user=/proc/$(cat $XDG_RUNTIME_DIR/libpod/tmp/pause.pid)/ns/user \
    --mount=/proc/$(cat $XDG_RUNTIME_DIR/libpod/tmp/pause.pid)/ns/mnt \
    /usr/local/bin/caddy run --environ --config /srv/caddy/Caddyfile"

Fixes: #21