Default config should be more persistent

Current Docker config does not have persistent storage configured, and may lose data unexpectedly. This issue is being opened to suggest adding default volumes with a docker-config like this:

```
services:
master:
container_name: "${COMPOSE_PROJECT_NAME:-citus}_master"
image: 'citusdata/citus:7.5.1'
ports: ["${MASTER_EXTERNAL_PORT:-5432}:5432"]
labels: ['com.citusdata.role=Master']
volumes:
- pgdata_master:/var/lib/postgresql/data
worker:
image: 'citusdata/citus:7.5.1'
labels: ['com.citusdata.role=Worker']
depends_on: { manager: { condition: service_healthy } }
volumes:
- pgdata_worker:/var/lib/postgresql/data
manager:
container_name: "${COMPOSE_PROJECT_NAME:-citus}_manager"
image: 'membership_manager_test'
volumes: ['/var/run/docker.sock:/var/run/docker.sock']
depends_on: { master: { condition: service_healthy } }
volumes:
pgdata_master:
pgdata_worker:
```