Dispatcher service check should escape DB_NAME with backticks

[gl]

In case the DB_NAME includes a special char, like "-", it is necessary to escape the database name with backticks. The following line should therefore be backslashbacktick${DB_NAME}backslashbacktick (sorry, github formatting is messing with the chars :))
Otherwise, the check fails and dispatcher service shuts down.

docker/rootfs/etc/cont-init.d/05-svc-dispatcher.sh

Line 73 in 0540fd9

while ! ${dbcmd} -e "desc $DB_NAME.poller_cluster;" >/dev/null 2>&1; do

[crazy-max]

SGTM, contribs welcomed