PhpBB doesn’t use Redis, MySQL has a smaller footprint than PostgreSQL and the overhead incurred by running a docker container is negligible.
Configuring phpBB on a VPS is probably a bit harder than installing a single Discourse instance. A tool à la Discourse’s docker launcher doesn’t exist, so at best you could get an Apache-MySQL-PHP (or Nginx-MySQL-PHP) stack as a Docker container which leaves you with the usual task of uploading phpBB’s files to your server by FTP, creating a database user and managing passwords and copying all that info into phpBB’s setup script…