Wait for Postgres to start up

[seanh]

We've been seeing the commands to create the databases failing on CI
with this error:

psql: error: connection to server at "localhost" (::1), port 5437 failed: server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.

The problem looks like a race condition between starting the Postgres
docker container and the command that creates the DB: we're trying to
create the DB before Postgres has finished starting up.

Fix this by setting Postgres docker container options that cause it to
wait for Postgres to be up and running before continuing.

See:

https://docs.github.com/en/actions/using-containerized-services/creating-postgresql-service-containers#running-jobs-in-containers