Get sshd to listen on multiple ports when systemd sockets are in use (affecting at least some recent Debian and Ubuntu containers)

I was going crazy trying to have sshd listen on multiple ports in a Debian Linux container under Proxmox Virtual Environment. This had been working at some point before on Debian Bookworm just by specifying multiple `Port` lines in /etc/ssh/sshd_config. But something changed at some point (either in Debian Bookworm updates or the upgrade to Debian Trixie) that changed how sshd is handled and caused it to appear to sometimes work and sometimes not.

I finally found this rude but helpful ServerFault answer:

https://serverfault.com/a/1142005/997178

It explains that sshd listen addresses and ports are now configured using systemd sockets. Setting them in sshd_config does nothing.

See /usr/share/doc/openssh-server/README.Debian.gz (use zcat) and pay special attention to the section near the end on systemd sockets.

Apparently this has been the default in Ubuntu for a while and recently became the pattern for Debian too.

Also see https://manpages.debian.org/stable/systemd/systemd.socket.5.en.html for info about sockets and the ListenStream option.

The final solution for me was to create `/etc/systemd/system/ssh.socket.d/listen.conf` containing:

[Socket]

#Clear ListenStream:

  ListenStream=

#Set new values. Multiple allowed:

  ListenStream=22

  ListenStream=2222