test: muuuuch easier local development

[vjeeva]

With this new code, we can simply run make local-dev to get the following:

1. Two Postgres DBs get spun up as Docker containers, one loaded with test data,
2. One container as a shell running a local version of belt

This way, if you need to test new features or commands, you can quickly test them against databases.

This builds and leaves around some docker containers. To clean up, run make clean-docker.