Such a wrapper script should call the command with exec function.

Otherwise, extra process will be created, and the script will not pass the
stdin to the sub process. (see man execv)
Also, $@ variable should be used with double quotes when you want to pass the
parameters as is.

slapos/recipe/generic_varnish/template/varnishlog.in

@@ -3,6 +3,4 @@
 DAEMON_OPTS="-a %(varnish_ip)s:%(varnishd_server_port)s \
              -n %(varnish_instance_name)s"
 
-# exporting PATH here so that it will pass the PATH variable to the subprocess
-export PATH
-%(varnishlog_binary)s ${DAEMON_OPTS} $@
+exec %(varnishlog_binary)s ${DAEMON_OPTS} "$@"