Do not download data when running

https://github.com/ckan/ckan-postgres-dev/blob/f8c1eca0853d82e3d1408bc613bcff47ead5ed65/docker-entrypoint-initdb.d/02_datastore_permissions.sh#L6-L13

Adding files should happen when building, not when running. How about using `ADD` in Dockerfile and avoid such `if` conditions to make things easier?

This would also make it possible to use non-alpine images, as regular Postgres images do not have `wget`, while the Alpine image has `wget` as an alias for `busybox`.