fix: correctly load Adminer plugins, fixes #41

[stasadev]

The Issue

• Plugins are not working with current adminer #41

We override the original index.php, which loads plugins.

Before (tables-filter is not loaded, unable to filter by table):

How This PR Solves The Issue

Uses ddev-adminer.php instead of index.php for PHP web server entrypoint, which allows to use the original index.php without modifications.

Manual Testing Instructions

ddev add-on get https://github.com/ddev/ddev-adminer/tarball/20250501_stasadev_plugins
ddev restart
ddev adminer

After (tables-filter is loaded, input is available):

Automated Testing Overview

Release/Deployment Notes