Add a docker configuration to ease testing

I was about to create a pull request to improve error handling when parsing the configuration when I noticed that the tests are running against all database type: pg, mysql, sqlite3 and mongodb

It's not very trivial for anyone to install and configure all these databases in order to test this library. I would recommend to create a `docker-compose` file that would take care of pulling the corresponding container and start them all.

I don't really have the time to look into it now, I'll have a look later this week if it's something that you would consider benefic.

Another solution would be to mock all the database drivers
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