Making solr available for manual testing

[christianwolters]

Dear all,

I read #69 and was happy, that solr is included - however:
it is not directly accesible for manual testing as the 8983 port is not exposed.

Exposing the port could be implemented or should at least be documented in the readme, so:

I could provide a PR introducing a new environment variable 'MOODLE_DOCKER_SOLR_PORT' with corresponding changes in the phpunit-external-services.yml and/or add another sections to the readme about this topic.

What would be the proper way here?

Best regards
Christian