Make sure flyway image has the sql/django_reference_data directory

[abachman-dsac]

follow-up to #178

Problem

The Flyway image doesn't contain the new flyway/sql/django_reference_data folder.

In development and CI it worked because backend/docker-compose.yaml mounts the whole flyway/sql folder as part of the db-migrations service definition.

    volumes:
      - '../flyway/sql:/flyway/sql'

Solution

Add ADD ./sql/django_reference_data ./sql/django_reference_data to the flyway deployment Dockerfile.

Result

The Flyway image built for deployment should include all required migrations.