Move CI checks toward Dockerfile workflow

[hokoo]

Summary

• keeps the two CI fixes split out from PR Support Ramsey Collection v1 and v2 #47 in their own branch
• makes Dockerfile-based CI the shared implementation for local and GitHub checks
• keeps GitHub Actions split into three independent workflows: PHP Code Styles, Unit Tests, and WP Integration Tests
• keeps local make tests.run as the aggregate development shortcut via test:all
• updates README badges/docs for the split checks

Docker entrypoint commands

• test:all installs dependencies and runs both test suites; this is used locally by make tests.run
• test:phpunit runs only the regular PHPUnit suite; this is used by the Unit Tests workflow
• test:integration runs only the WordPress-backed integration suite; this is used by the WP Integration Tests workflow
• cs:phpcs installs Composer dependencies and runs the existing composer run phpcs

Validation

• bash -n docker/phpunit-entrypoint.sh
• docker build --build-arg PHP_VERSION=8.1 -t wpconnections-ci:8.1-local -f Dockerfile.phpunit .
• Docker test:all on PHP 8.1 with RAMSEY_VERSION=^1
• Docker test:all on PHP 8.1 with RAMSEY_VERSION=^2
• Docker cs:phpcs on PHP 8.1
• cd local-dev && docker-compose -p wpconnections build phpunit
• make tests.phpunit
• make tests.integration
• make tests.run
• make lint.phpcs
• GitHub Actions on d59a4f3: PHP Code Styles passed, Unit Tests passed, WP Integration Tests passed