Feature Request: virus scanning / clamav

[t-lo]

This is a good first issue to start with if you'd like to become a contributor to the mailserver project.

Add virus scanning to the mailserver, e.g via clamav or other virus scanners available on Alpine. The feature should be added directly to the mailserver image, i.e. no external container should be used.

The virus scanner should be integrated directly with postfix, e.g. via postfix' content_filter feature.