Update base image to PHP 8.2-fpm-alpine

[Sama34]

This pull request updates the base PHP version used in the Dockerfile from 7.4 to 8.2. This is a significant upgrade that will enable access to newer language features, improvements, and security patches.

Environment update:

• Dockerfile: Changed the base image from php:7.4-fpm-alpine to php:8.2-fpm-alpine to upgrade the PHP runtime.

[Copilot]

Pull request overview

Updates the Docker base image to run MyBB on PHP 8.2 (Alpine) instead of PHP 7.4, modernizing the runtime for security updates and newer language/runtime behavior.

Changes:

• Bump Docker base image from php:7.4-fpm-alpine to php:8.2-fpm-alpine.

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