Shorten default cert duration for compatibility and security

Some software and organizations (specifically Apple) are moving to default lack of trust for certs with long duration.

Apple's Safari (and perhaps other software from Apple) on iOS 18 will default to not trust a certificate if the duration is longer than 825 days.

Recommend changing

`CERT_DURATION="-days 1825"`

to

`CERT_DURATION="-days 800"`

or something similar.

I would put in a pull request or whatever its called but I'm not knowledgeable about that.

Also, FWIW, thanks for the container and script. Still working well 9 years after the last update.

Search the repository for the CERT_DURATION="-days 1825" definition and inspect the script or configuration that uses it. Change the default to a duration at or below the requested compatibility limit, then verify that certificate generation uses the new default.

Written by the indexing model from the issue text.