Additional docker compose file for creating a vanilla lein repl

I've created an additional vanilla lein repl task in an optional docker-compose file. This makes it possible to do the following:

docker-compose -f docker-compose.yml -f optional-docker-compose.yml up repl

This is useful when you want a headless repl that you can connect local tooling to.

I've included it in an optional docker-compose file so that it doesn't cause any issues for existing workflows.

I've bumped the docker compose file version as part of this, it doesn't look to have caused any problems.