fix: Avoid duplicate entries on /etc/postfix/header_checks on container restart

[juanluisbaptiste]

Description of the change

Fixes avoiding duplicate entries in /etc/postfix/header_checks file as PR #129 was trying to do, but in a more generic way that cover any other case where that file was or could be modified.

Motivation and Context

Fixes the creation of duplicate entries in /etc/postfix/header_checks when the container is restarted.

Types of Changes

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

Checklist:

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