This feature was introduced by the PR: docker-mailserver#1463

There is no official documented support for custom DH parameters. As no guarantee is provided, this is considered an internal change, not a breaking one.

There is no apparent need for special handling with `ONE_DIR=1`.

This feature was originally introduced by the PR: #1463

- Assign default DH params to use via Dockerfile build instead of copy and update at runtime.
- Parameterized service names and paths.
- Refactor postfix and dovecot dh methods to wrap shared dh logic
- I don't see any value in checking the alternative service for dh params file to copy over, so that's now dropped too.
- Another conditional check is dropped and the default fallback message for existing DH params file is no longer relevant.
- Improved the remaining `_notify` messages. Collapsing the warning into a single logged message also seemed relevant.
- There is no apparent need for special handling with `ONE_DIR=1`. Dropped it.

- Refactor DH params  tests
- Combine custom and default DH param tests into single test file
- docs: Add instructions to use custom DH params

There is no official documented support for custom DH parameters. As no guarantee is provided, this is considered an internal change, not a breaking one.