Feature Request: Explicitly define a internal network in docker-compose.

## Summary

This is a request to explicitly define docker networks in the docker-compose.

## Motivation

For the purposes of adding sentry onpremise to existing networks, some containers might need to be added to a external network. This will prevent that container from accessing the default network created by docker-compose. To prevent this from happening, all containers need to be explicitly added to a network that they share with all other containers they need access to.

This is important for cases where users may want to reverse-proxy sentry themselves, especially using something like traefik.

related to #698