Add systemd to boot of a docker container

I have an ubuntu:22.04 container in docker. I need to access to systemd on it. Now, I've installed the systemd via my Dockerfile. But after running the image, systemctl command produces following error:

root@d0184b7fe9ce:/var/www# systemctl
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to bus: Host is down

What should I do to solve the problem?

Why I need systemctl?
I'm trying to install php8.2 on my container. After a successful installation, I have no php and php-fpm socket in /var/run/. I think it's because of that the systemctl is not working.

This isn't how docker works and is certainly not how it's supposed to work here.

In the very roughest terms, a docker-run process is:

• A context made up of:
  • A filesystem (made up of various volume images mounted over each other, and configuration-time mounts onto your system over the top of those)
• A dockerfile saying what to run in that context.

You layer on all sorts of other configuration but it's just a process tree running in a separated context under your kernel. It's not a full system, it's not a VM that not "boots". An init system would completely obviate the thin hypervisor model.

So pull this back to "I have an ubuntu:22.04 container in docker". That's nice but Ubuntu there is incidental to PHP. If you want PHP, run the PHP image and use its configuration to tie in your filesystem (eg the PHP you want to serve) or to expose the FPM server to the webserver (wherever that's running).

Chances are you've also got a database and a httpd. You might benefit from writing a docker-compose file that carries all these functions (from different images) and ties them together. The result will be several process trees running together, each fairly isolated but still centrally controllable. And great if you ever need different versions of things for different sites.

But I think you'd be wrong to try to treat a base image of Ubuntu as a deployable. You could —you absolutely could— but it's not the point of docker. If you want a bare-metal-like configuration, create a proper VM with KVM or XEN.