Old phpunit not support to export cov report file so you can't merge them with phpcov tool This will help to merge report file in xml file format.
docker build . -t merge-xml-report
And run it with this
docker run -v "<path_to_your_result_dir>:/files/result" -v "<path_to_your_reports_dir>:/files/reports" merge-xml-report
Log in docker hub of Hamee
docker login
And run it with this
docker run -v "<path_to_your_result_dir>:/files/result" -v "<path_to_your_reports_dir>:/files/reports" trinhtin/merge-xml-report:latest