Add php8.1-bcmath extension to fix ScrambleMapping test failure

[tosfos]

This PR fixes a PHPUnit test failure in the CI pipeline by adding the php8.1-bcmath extension to the Dockerfile.

Problem

The MediaWiki 1.43 ScrambleMapping class requires either the bcmath or gmp PHP extension. The current Dockerfile was missing both extensions, causing this PHPUnit test error:

MediaWiki\Tests\User\TempUser\ScrambleMappingTest::testOffsetTooLarge
RuntimeException: MediaWiki\User\TempUser\ScrambleMapping requires the bcmath or gmp extension

Solution

Added php8.1-bcmath to the list of PHP extensions installed in the Docker image. The bcmath extension provides arbitrary-precision mathematics functions needed by the ScrambleMapping class.

Testing

This change will be tested by the existing CI pipeline which runs PHPUnit tests against MediaWiki core.

[tosfos]

The issue was found while integrating PHPUnit tests in #46 . Before this change, I got:

MediaWiki\Tests\User\TempUser\ScrambleMappingTest::testOffsetTooLarge
RuntimeException: MediaWiki\User\TempUser\ScrambleMapping requires the bcmath or gmp extension
/var/www/mediawiki/w/includes/user/TempUser/ScrambleMapping.php:74
/var/www/mediawiki/w/tests/phpunit/unit/includes/user/TempUser/ScrambleMappingTest.php:37

After this change, this test was no longer failing.

[github-actions]

🐳 The image based on 57103fe7 commit has been built with 1.43.6-20251224-47 tag as ghcr.io/canastawiki/canasta-base:1.43.6-20251224-47