Add a postgres database to the docker-compose.yaml for UC server

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To finish making the experience amazing, we'll remove any issues caused with ephemeral database processes on the UC server. This way when we create new tables or work with our data we will have a consistent experience (even if we shut down the docker environment)

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In this PR, we should also add more to the README to tell people the exact steps to run the environment, including opening up marimo in the docker environment.