Add secure generation and display of initial user credentials

Add logic to `start.sh` to generate and display initial admin and user passwords, and update `docker-compose.yml` to use these generated passwords.

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docker-compose.yml

@@ -27,6 +27,8 @@ services:
     environment:
       NODE_ENV: production
       DATABASE_URL: postgres://${POSTGRES_USER:-tx}:${POSTGRES_PASSWORD:-tx_dev_password}@postgres:5432/${POSTGRES_DB:-tx}
+      SEED_ADMIN_PASSWORD: ${SEED_ADMIN_PASSWORD:?SEED_ADMIN_PASSWORD is required — re-run ./start.sh to generate one}
+      SEED_USER_PASSWORD: ${SEED_USER_PASSWORD:?SEED_USER_PASSWORD is required — re-run ./start.sh to generate one}
     user: root
     entrypoint: ["/usr/bin/tini", "--"]
     command: ["/bin/bash", "/usr/local/bin/migrate.sh"]