Define separate Dockerfiles for Postgres and Make services

[jeancochrane]

Overview

This PR splits db/Dockerfile into two Dockerfiles so that the Postgres, Make, and Django services can each have their own Dockerfile with the separate dependencies required of each service.

Closes #17.

Testing instructions

• Make sure you don't have any new mapping data; if you do, save it to a fixture first by following the Exporting routes section of the README
• Remove your development database with docker-compose down --volumes
• Follow the new instructions in the Developing section of the README and make sure that the commands successfully build containers and import data