[Olefy] Update to Alpine 3.15 and include ping fix

This PR is updating the olefy-mailcow container to Alpine 3.15.

It is also including the ping fix from @a16bitsysop which is solving the issue: #4401

The PR includes a temporarily local copy of the olefy.py file which includes the named fix. When this fix is officially merged into the olefy project we will update the olefy container again.

The new docker image tag is: mailcow/olefy:1.8.1