Bump Dockerfile base image to php:8.4-apache

[mahmoodhamdi]

Summary

Bumps the Dockerfile base image from php:7.3-apache to php:8.4-apache so
that docker compose up --build actually completes on master.

Why

The current base image installs PHP 7.3, but:

• composer.json requires "php": ">=8.3.0"
• composer.lock pins symfony/var-dumper v8.0.8 and symfony/yaml v8.0.10,
  both of which require PHP >= 8.4
• yiisoft/yii2 2.0.54 requires PHP >= 7.4.0

So composer install aborts inside the container on a fresh clone of
master, taking down the documented "Quick start" path with it.

Full context and reproducer in #165.

Verification

Verified locally that the one-line change makes the build complete
end-to-end:

• composer install finished in 28.5s with all 57 packages installed
• The image exported successfully
• No other Dockerfile changes were needed — the pdo, pdo_mysql, and
  mysqli PHP extensions and Apache's rewrite module install the same
  way on the new base

Risks

PHP 8.x is a substantial jump from 7.3, but:

• The composer.json already declares php >= 8.3.0, so the application
  is expected to run on PHP 8.x regardless of the Dockerfile.
• Recent commit history includes "More PHP 8 migration changes" and related
  fixes, suggesting the codebase is already PHP 8.x-aware.
• This change only affects the dev/Docker build path; production deployment
  is configured separately.

If you'd prefer a different target version (e.g. php:8.3-apache), happy to
update — 8.4 is the smallest version that satisfies every constraint in
composer.lock without touching dependencies.

Fixes #165