There was a typo where we said we were creating the containers but we
were building the image.
While none of these commands are necessary because they're run
automatically when first running the `db:create` rake task, I think it's
better to show them separately, since the `build` command might be used
to rebuild the image, and running the `create` command separately makes
it easier to know what's going on.
We're also removing the "start database service" step, since it's
automatically started because the app service depends on it.
Finally, we're updating one command for macOS.
d88ea7f26d