In Debian Bullseye, the Debian package creates /etc/service as a symlink to
/etc/runit/runsvdir/current. The /etc/runit folder is a byzantine complex
construction with for example a "current" and "default" set of services.
This change switches to an independent /var/service folder, where images
can maintain a simpler structure.
This fixes issue to build the image using BuildKit, as this mechanism
doesn't allow to replace a symlink by a directory.
Ref T1679.