Document how alembic is run in production via the containers

Extending #44 we should define how `alembic` is configured in the production container and how you are expected to run the migration in production environments.

We should also think about providing [Task](https://taskfile.dev) based scripts, Tasks is available as a [Github action](https://taskfile.dev/installation/#github-actions) which would work well with the proposed CI/CD workflow.