[Etherpad] Fix bad syntax in entrypoint

pica-etherpad/entrypoint.sh

@@ -22,7 +22,7 @@ if [ -z "${DB_HOST}" ]; then
 	exit 1
 fi
 
-while ! { mysqladmin ping -u${DB_USER} -p${DB_PASSWORD} -h"${DB_HOST}" &>/dev/null || PGPASSWORD=${DB_PASSWORD} psql -h${DB_HOST} -U${DB_USER} -c "SELECT 1" }; do
+while ! { mysqladmin ping -u"${DB_USER}" -p"${DB_PASSWORD}" -h"${DB_HOST}" &>/dev/null || PGPASSWORD="${DB_PASSWORD}" psql -h"${DB_HOST}" -U"${DB_USER}" -d"${DB_NAME}" -c "SELECT 1" &>/dev/null; }; do
 		echo "Database server not ready yet, re-trying in 5 seconds..."
     sleep 5
 done