Use a password generator for secure passwords

Passwords are hardcoded in the `.env` file which is then used by docker compose to populate environment variables used in the running containers. These passwords are therefore pretty insecure. Therefore I propose to create a process to generate random secure passwords before the local images are built and before the containers start

The following environment variables will use the passwords generated

1. `POSTGRES_PASSWORD`
2. `CKAN_DB_PASSWORD`
3. `DATASTORE_READONLY_PASSWORD`
4. `CKAN_SYSADMIN_PASSWORD`

The passwords will be housed in a newly generated file which will have it's permissions locked down to just read-only by the file owner. This file (named `.pw`) can then be added to both ckan and db services in the `docker-compose.yml` file as follows: