I did this for free codecamp:
volumes:
- volume:
host: /var/discourse/shared/standalone
guest: /shared
- volume:
host: /var/discourse/shared/standalone/log/var-log
guest: /var/log
- volume:
host: /var/postgres_data
guest: /shared/postgres_data
Basically I keep /var/postgres_data
in root and then mount /var/discourse/shared/
on the second volume, additionally I put docker containers in /var/discourse/shared/docker
There are many ways of skinning this, just keep postgres_data on the fastest drive and you should be good.