Add support for CLOUDFLARE_PROPAGATION_SECONDS environment variable

[lmerega]

What does this PR do?

This PR adds support for the CLOUDFLARE_PROPAGATION_SECONDS environment variable,
which is passed as an optional argument to Certbot:

--dns-cloudflare-propagation-seconds

If the variable is not defined, the argument is omitted and Certbot uses the default value (10 seconds).

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Why is this useful?

In some cases, DNS TXT records may take longer than 10 seconds to propagate, depending on DNS caching or specific configurations on Cloudflare.
Allowing the propagation time to be configured helps avoid issues during domain validation with Let's Encrypt.

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How to use

Simply define the variable when running the container, for example:

docker run -e CLOUDFLARE_PROPAGATION_SECONDS=30 ...

This will make Certbot wait 30 seconds before attempting domain validation.

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Notes

• If the variable is unset or empty, the parameter is omitted entirely.
• Default Certbot behavior (10s wait) is preserved.