feat: Add override for origin domain

[cgraf-spiria]

Description of the change

Adding override for origin domain.

Motivation and Context

In my case, my sending domain is foo.bar.com but the current script only keeps bar.com and my relay SMTP refuses the email. By allowing to override the default behaviour, I can set the origin domain to the correct value.

Fixes issue: https://github.com/juanluisbaptiste/docker-postfix/issues/84

How Has This Been Tested?

Simple bash "if value is not defined" it does the previous behaviour.

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation (adding or updating documentation)

Checklist:

• [x] My change requires a change to the documentation and I have updated the documentation accordingly.
• [x] My change adds a new configuration variable and I have updated the .env.example file accordingly.