For the Unix domain socket over which haproxy and varnish communicate,
we have chmod 660 and chgrp varnish. haproxy belongs to group varnish
and thus has write permissions on the socket, which is required in
Linux to be able to connect.

For that, both containers must have the same group name and GID.
We've been using 998, since that results from the RPM install for
Varnish from the official packagecloud repos.