fix: Ensure correct ownership for the Rspamd DKIM directory

[polarathene]

Description

Applies the suggested fix from reporter: #3800 (comment)

• The UID / GID shifted during a new release.
• Until DKIM handling is refactored in a new major release, this fix ensures the content maintains the expected _rspamd ownership.

Fixes #3800

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OpenDKIM recently had a contribution for ownership adjustment, although that was for the user benefit with config volume files, not an issue with DMS support.

Ownership issues in /tmp/docker-mailserver shouldn't matter or need to be fixed for DMS, but the present rspamd DKIM support has a rather relaxed / flexible location where it's associated config points to the files instead of a local internal copy made at runtime.

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NOTE: There is already related script for rspamd during startup that attempts to get DKIM keys to inspect by parsing path settings in dkim_signing.conf, logging warnings when the ownership is not _rspamd.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• I have added information about changes made in this PR to CHANGELOG.md

[georglauterbach]

I will adjust the changelog with the v13.3.1 PR to account for this.