feat: add fileinfo, pdo, pdo_mysql, pdo_pgsql, simplexml to all PHP versions

[josecarlospeer-cloud]

Closes #102 (extensions item)

What

Adds four commonly required extensions missing from all PHP images:

Extension	Why it matters
fileinfo	Required by Laravel, Symfony for MIME type detection; missing breaks file uploads
pdo	Base PDO interface — prerequisite for pdo_mysql / pdo_pgsql
pdo_mysql	MySQL/MariaDB via PDO — the most common PHP database driver
pdo_pgsql	PostgreSQL via PDO — required by this repo's own reference project
simplexml	XML parsing; implicit dependency of several composer packages

Scope

All four PHP versions: 8.1 (alpine:3.19), 8.2 (alpine:3.21), 8.3 (alpine:3.21), 8.4 (alpine:edge).

Verification

Package existence confirmed in Alpine repos before adding:

alpine:3.19  php81-fileinfo, php81-pdo, php81-pdo_mysql, php81-pdo_pgsql, php81-simplexml ✅
alpine:3.21  php83-fileinfo, php83-pdo, php83-pdo_mysql, php83-pdo_pgsql, php83-simplexml ✅

Build-tested locally (8.3 main stage):

$ php -m | grep -E 'fileinfo|PDO|pdo_mysql|pdo_pgsql|SimpleXML'
fileinfo
PDO
pdo_mysql
pdo_pgsql
SimpleXML  ✅