Improve Docker Compose definitions with databases

[Chuxel]

This PR resolves #504, #505, #506, and #507 to the Docker Compose-based definitions that include a database by:

1. Adding in a database extension, and if possible (Postgres, MariaDB) pre-configuring it
2. Adding comments into devcontainer.json and docker-compose.yml about where the database can be found
3. Adding comments to docker-compose.yml about how to expose the database port
4. Updating the READMEs with more details on the database defaults

//cc: @bamurtaugh @burkeholland @chrmarti

[bamurtaugh]

LGTM, thanks for putting this together, Chuck!