fix: PROXY protocol should share postscreen cache

[polarathene]

Implements a fix based on findings at: #186 (comment)

This was observed earlier back in April 2025 and pointed out as not aligned with our docs guidance. Managing the setting via user-patches.sh as I've contributed here is better than the current DMS docs advice with postfix-main.cf, so I'll update that over there too.

This approach to share the postscreen cache is motivated from this DMS issue.

• Technically depending how it's leveraged privately, perhaps there could be some overlap with public entries in the cache that could be exploited?
• Without further investigating how the cache is managed and putting more thought into it, I'm not able to weigh in much on that concern, or any other risks.

Should it be an issue, separate cache maps can be configured and proxy_write_maps should be updated as per #186 (comment)

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Fixes: #186

[rich0]

I can confirm that this works for me running chart version 4.2.4 with this change to user-patches.sh added to my values.yaml, over my traefik-ingress proxy.

[cfis]

Great - thanks you both!