Rewrite action to Docker container + JS Testcontainers tests

[MekDrop]

Summary

• rewrite the action from composite to Docker container action
• use official PHP base image with build arg (PHP_BASE_IMAGE) so image can be built for legacy PHP lines (validated with php:5.5.38-cli and php:7.4-cli)
• stop using Composer package install flow in the action runtime; clone evert/phpdoc-md from git in the Docker build stage and bundle its runtime deps
• download phpDocumentor artifacts at runtime (no phar committed in repo), preferring release .tgz for versioned builds and falling back to .phar
• replace Bats tests with JS integration tests based on Testcontainers that build/run the action container for multiple PHP versions and verify markdown generation
• update PR workflow to run
  pm ci +
  pm test

Validation

pm test (passes locally)