Fix SAIL_SHARE_DOMAIN default value

[gonzalom]

This pull requests provides a default value for SAIL_SHARE_DOMAIN fixing the issue #545

If the value of SAIL_SHARE_DOMAIN is not set, it will get the value from SAIL_SHARE_SERVER_HOST, which by default is laravel-sail.site, recovering the original behaviour in version 1.18.1, before the commit 8c9d836, and therefore fixing the issue mention above where the public shared hostname has only the autogenerated subdomain.

$ sail share
Thank you for using expose.
Shared URL:		host.docker.internal:80
Dashboard:		http://127.0.0.1:4040
Public HTTP:		http://ahirytx7qj.:8080
Public HTTPS:		https://ahirytx7qj.