Add docker-compose for controlled local development

[olvlvl]

Q	A
Type	feature
BC Break	no

Hi,

This PR adds Docker support to help with local development.

Run make test-container to create the container and its MongoDB dependency. A terminal is opened once the container is ready, run make test to run the test suite. When you're done with the tests just enter exit and the container will be destroyed and it's dependency shutdown.

The container image is built from the official PHP 7.2 image, and includes the MongoDB driver, the bcmath extension, and Composer (to install packages from within the container).

I also added ext-bcmath to composer.json since the extension is required by the AlnumGenerator class, but I didn't indicate a version because I didn't know which one to pick.

I hope this helps.

[alcaeus]

After internal discussion, we've decided not to merge this. None of us use docker for our development environments, so maintaining the infrastructure would be an additional chore with no extra gain.

As for the bcmath extension, while technically required for AlnumGenerator, we've intentionally left it out since it's only required if you actively specify the generator in the mapping. Instead of a hard requirement, this should be checked by the class itself, with the extension put into a suggest section in composer.json to explain where it's used. Please create a separate pull request for that if you're interested in making the change.