Move to PHP 8.4 and fix the xdebug build

[Firehed]

PHP 8.1 is EOL, and the Docker build was failing independently of that. This moves the template to 8.4 and fixes the build.

Two unrelated breakages

xdebug no longer compiles.pecl install xdebug fails at configure with rtnetlink.h is required, install the linux-headers package. A recent Xdebug release added that dependency, and $PHPIZE_DEPS does not include it. This is not version-specific — it reproduces identically on 8.1 and 8.4. linux-headers is installed in the env-with-xdebug stage only, so the production env and server images stay unchanged.

The 2023 dependency set does not run on 8.4.nikic/php-parser 4.x triggers an implicitly-nullable-parameter deprecation, which bootstrap.php promotes to an ErrorException, so orm:generate-proxies fatals during the server build. A plain composer update resolves it — every package stayed inside its existing constraint, so no major version bumps were required here. It also clears the security advisories that composer audit was reporting.

Base image tag

php:8.1.0-alpine3.15 becomes php:8.4.24-alpine, dropping the Alpine pin.

The pin is why #62 offered only 8.1.0 to 8.1.13: Dependabot preserves the tag suffix, and Alpine 3.15 has no PHP builds past 8.1, so the upgrade path dead-ended. Without the pin, Dependabot can move PHP across minors and majors, and the base picks up Alpine security rebuilds.

Effect on the open Dependabot PRs

• Bump php from 8.4.24-alpine to 8.5.9-alpine #62 (php 8.1.13) and Bump vlucas/phpdotenv from 5.3.1 to 5.6.4 in the composer-minor-and-patch group #60 (phpdotenv, via the composer group) are superseded by this branch and can be closed.
• Bump actions/checkout from 3.1.0 to 7.0.1 #61, Bump docker/login-action from 2 to 4 #63, and Bump codecov/codecov-action from 3 to 7 #64 (Actions bumps) are unaffected.
• Bump mysql from 8 to 26 #59 (mysql 8 to 26) is unrelated and being handled separately; 26 is an innovation release rather than LTS.