Dockerfile: Remove not needed ENVs and add comment

[casperklein]

Description

This PR removes not needed ENVs and adds a comment to clarify, why the other ENVs are needed.

These ENVs are not needed, they are handled by start-mailserver.sh like all other ENVs in DMS:

• ONE_DIR=1
• ENABLE_POSTGREY
• SASLAUTHD_MECHANISMS

The remaining ENVs are referenced in target/supervisor/conf.d/saslauth.conf and must be present when supervisord starts (tested).

If necessary, their values are adjusted by start-mailserver.sh on startup.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[georglauterbach]

👍🏼

[wernerfred]

LGTM thx