Update Postgres instructions for Rails Testing

[mheap]

The default Postgres image has changed and without any env variables the healthcheck will never pass.

You need to set POSTGRES_USER and POSTGRES_DB, and may choose to set either POSTGRES_PASSWORD or POSTGRES_HOST_AUTH_METHOD. If you choose to set a password, your Rails test config must specify that password. Alternatively, you can set POSTGRES_HOST_AUTH_METHOD=trust to allow all local connections without a password.

See https://github.com/docker-library/postgres/commit/42ce7437ee68150ee29f5272428aa4fc657dc6dc for more context