container: T6702: re-add missing UNIX API socket (#4039)

During podman upgrade and a build from the original source the UNIX socket
definition for systemd got lost in translation.

This commit re-adds the UNIX socket which is started on boot to interact with
Podman.

Example:

curl --unix-socket /run/podman/podman.sock -H 'content-type: application/json' \
  -sf http://localhost/containers/json

(cherry picked from commit f67e217f2716937115a3bdf6d316b172bbec75e5)

Co-authored-by: Christian Breunig <christian@breunig.cc>