Add an example with PostgreSQL and Docker Compose

[aantron]

...to serve as an easy starting point for people that want that kind of setup.

We already have an example that just uses esy, Dream, and Docker Compose. We just need to:

• Copy it and add a Postgres container to it.
• Connect to the database from the Dream code. See the SQL example. We need to do something similar, but use a Postgres connection string rather than SQLite.
• Remove deployment boilerplate, as it won't be a deployment example. We can link back to the Docker deployment example for that.