Update CI Docker run_tests to test against PostgreSQL

The tests inside the Docker build job of the CI/CD pipeline run against an SQLite database.

Since PostgreSQL is the reference database for production, the Docker build job (https://gitlab.com/mayan-edms/mayan-edms/-/blob/master/.gitlab-ci.yml#L23) needs to be updated to launch a PostgreSQL container of the supported version (https://gitlab.com/mayan-edms/mayan-edms/-/blob/master/config.env#L17) and pass the environment setting to the Mayan container executing the run_tests command.