Systemd-nspawn single process

[Warex61 YTB <thomasdabout61@xxxxxxxxx>]

Hello, 

I would like to use systemd-nspawn to create a container that can launch a single process as pid 1 and mount its configuration files. I want the container to be as light as possible. Is there any way of creating a container using nspawn without using bootstrap ?

For example, using this command, without using a bootstrap

systemd-nspawn -M process -D /etc/systemd/nspawn/process /etc/systemd/nspawn/process.nspawn

I get the following error

Directory /etc/systemd/nspawn/process doesn't look like it has an OS tree. Refusing.
What are the conditions for nspawn to consider an OS tree in /etc/systemd/nspawn/process ?

Thanks.