fix: Enable DH parameters (ffdhe4096) by default (#2192)

This feature was originally introduced by the PR: https://github.com/docker-mailserver/docker-mailserver/pull/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.

docs/content/config/security/ssl.md

@@ -654,6 +654,12 @@ if [ "$certcheck_2weeks" = "Certificate will not expire" ]; then
 fi
 ```
 
+## Custom DH Parameters
+
+By default `docker-mailserver` uses [`ffdhe4096`][ffdhe4096-src] from [IETF RFC 7919][ietf::rfc::ffdhe]. These are standardized pre-defined DH groups and the only available DH groups for TLS 1.3. It is [discouraged to generate your own DH parameters][dh-avoid-selfgenerated] as it is often less secure.
+
+Despite this, if you must use non-standard DH parameters or you would like to swap `ffdhe4096` for a different group (eg `ffdhe2048`); Add your own PEM encoded DH params file via a volume to `/tmp/docker-mailserver/dhparams.pem`. This will replace DH params for both Dovecot and Postfix services during container startup.
+
 [docs-optional-config]: ../advanced/optional-config.md
 
 [github-file-compose]: https://github.com/docker-mailserver/docker-mailserver/blob/master/docker-compose.yml
@@ -661,4 +667,8 @@ fi
 [hanscees-renewcerts]: https://github.com/hanscees/dockerscripts/blob/master/scripts/tomav-renew-certs
 
 [traefik::github]: https://github.com/containous/traefik
-[ietf::rfc::acme]: https://tools.ietf.org/html/rfc8555
+[ietf::rfc::acme]: https://datatracker.ietf.org/doc/html/rfc8555
+
+[ietf::rfc::ffdhe]: https://datatracker.ietf.org/doc/html/rfc7919
+[ffdhe4096-src]: https://github.com/internetstandards/dhe_groups
+[dh-avoid-selfgenerated]: https://crypto.stackexchange.com/questions/29926/what-diffie-hellman-parameters-should-i-use