Correct entrypoint.sh causing bootloop in some cases (#1628)

* Correct bootloop in some cases.

Lines "kill -15 $SUPERVISOR_PID" then "wait $SUPERVISOR_PID" may fail if process is killed sufficiently fast. Command "ps -p" ensure process is still here before waiting to its state to change.

* Less output

entrypoint.sh

@@ -19,7 +19,7 @@ case ${1} in
         SUPERVISOR_PID=$!
         migrate_database
         kill -15 $SUPERVISOR_PID
-        wait $SUPERVISOR_PID
+        ps h -p $SUPERVISOR_PID > /dev/null && wait $SUPERVISOR_PID
         rm -rf /var/run/supervisor.sock
         exec /usr/bin/supervisord -nc /etc/supervisor/supervisord.conf
         ;;