This refactor simplifies the `Dockerfile` by considerable amount:
new features of mobykit buildkit allow us to cache `apt`/`apk`
folder (so no need to delete `/var/lib/cache`) as well as extract
the build / install logic into separate files, which removes the
need for `&& \` and makes the code *much more* readable.
`build.sh` script has been updated as well to allow caching local
builds when testing, making it much more easier to test.
This new feature will anonymize emails in the Postfix logs. This allows
you to keep them indefinetely while being compliant with GDPR.
Based on excellent work on [this pull request](https://github.com/bokysan/docker-postfix/pull/91).
Check `README.md` for more details.
