test: move mongodb to different port to avoid port conflicts (#14993)

This moves the exposed mongodb docker image port from 27017 to 27018 in
order to avoid conflict with locally installed mongodb applications
running on port 27017.

Previously, you would have to make sure you don't have mongodb running
locally when using the docker image - otherwise, it would fail to
connect.

It also adds the old .env variables commented out in the .env.example
for users who do not wish to use docker. The CONTRIBUTING.md now
includes docker installation steps for macOS users.