Add documentation for the Docker Compose file

# 💡 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 https://github.com/cisagov/publish-egress-ip-lambda/pull/4#discussion_r1608714477 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.