chore: use postgres and update quickstart guide

[pscott]

Closes: https://github.com/snapshot-labs/workflow/issues/425

This PR moves from the old mysql to postgres.
It:

• Updates the CI to use postgres
• Updates the docker-compose file to use postgres
• Uses the default values of docker-compose directly into .env.example so that a user can just run cp .env.example .env and it will work
• Updates the README to include the yarn build command before running yarn start

[Sekhmet]

utACK

[Sekhmet]

Feel free to merge.