feat: Install PHP 8.1

[bakerkretzmar]

Adds PHP 8.1 to the build image. Just like 7.4, it can be selected and used in builds by setting the PHP_VERSION environment variable.

PHP 8.1 was released almost a year ago, and PHP 8.0 (the highest version currently included in this build image) will stop receiving active support next month when 8.2 is released.

See https://www.php.net/supported-versions.php

To test this I built and ran the image locally and verified that PHP 8.1 is installed and runs.

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[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/

[intrepidws]

As PHP 7.4 has been EOL'd and PHP 8.2 is now out, I wanted to check in on the status of a build with PHP 8.1. Are there any plans to release this?

[kitop]

We're looking into this and aiming to release it soon!