Thank you @flamber 
For anyone trying to setup metabase with a PG db with docker, the following worked for me:
version: '3.3'
services:
db:
image: postgres
restart: always
environment:
- POSTGRES_PASSWORD=PASWORD
- POSTGRES_USER=USERNAME
- POSTGRES_DB=DATABASE
ports:
- 5432:5432
volumes:
- ./db-data:/var/lib/postgresql/data
networks:
- postgres-network
metabase:
volumes:
# setup your SQLITE db (optional)
- /PATH/TO/SQLITE/:/metabase
- ./metabase-data:/metabase-data
environment:
- MB_DB_FILE=/metabase-data/metabase.db
- MB_DB_TYPE=postgres
- MB_DB_DBNAME=DATABASE
- MB_DB_PORT=5432
- MB_DB_USER=USERNAME
- MB_DB_PASS=PASWORD
- MB_DB_HOST=db
ports:
- 3000:3000
image: metabase/metabase
restart: always
depends_on:
- db
networks:
- postgres-network
networks:
postgres-network:
driver: bridge