This file only exists at the location to be copied to separate `postfix` and `dovecot` locations. `/etc/postfix/shared` seems like an inappropriate location to have been using.

These are effectively the same, did a diff comparison:

- Identified minor inconsistencies and in `_notify` output.
- Parameterized service names and paths.
- Added some comments to better grok the flow.

Next these two methods can be collapsed into a single one with input args to make it DRY.

This only needs to modify the `DH_CUSTOM` path var. More DRY, the conditional branch for `ONE_DIR=0` (else), is kept and shifts it's indentation one level to the left.

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.

Custom provided DH params now use `cp -f` to overwrite the existing default `ffdhe4096.pem` used.

- Normalize whitespace (had mixed indentation of spaces and tabs, and mixed indentation width of 2 and 4 spaces).
- Improved formatting of docker args.
- (default test file) Swapped order of ONE_DIR bool definitions, and `PRIVATE_CONFIG` var duplicated with ONE_DIR suffix instead of recycling the var.
- Updated the grep tests checking docker logs for warning message.

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`.

Now DRY and ready to merge the other two DH param tests into.