Add documentation for the Docker Compose file

[mcdonnnj]

💡 Summary

It would be helpful to add some documentation that explains the configuration options of the Docker Compose file used to generate Lambda artifacts (or run the Lambda locally).

Motivation and context

This was suggested in cisagov/publish-egress-ip-lambda#4 (comment) and would generally be useful.

Implementation notes

Modifying the docker-compose.yml file itself should be mentioned but so should overriding things on the command line.

Acceptance criteria

How do we know when this work is done?

• The project's README provides information on the Docker Compose file that is used to build Lambda artifacts.