Make sure PHPCompatibility is registered with PHPCS

Author: jissereitsma

magento-coding-standard/Dockerfile:7.4

@@ -5,6 +5,7 @@ COPY --from=composer:1 /usr/bin/composer /usr/local/bin/composer
 RUN /usr/local/bin/composer global require magento/magento-coding-standard:*
 RUN /usr/local/bin/composer global require phpcompatibility/php-compatibility:*
 RUN ~/.composer/vendor/bin/phpcs --config-set installed_paths ../../magento/magento-coding-standard/
+RUN ~/.composer/vendor/bin/phpcs --config-set installed_paths ../../phpcompatibility/php-compatibility
 RUN echo memory_limit = -1 >> /usr/local/etc/php/conf.d/custom-memory.ini
 
 ADD problem-matcher.json /problem-matcher.json