For the mattermost container, there needs to be an update to the template:
First off, they are moving away from mysql and pushing their users to use postgres, so it would be wise to mention this, and have the default first setup be postgres.
Second the template for DATASOURCE is formatted for mysql:
MM_USERNAME:MM_PASSWORD@tcp(DB_HOST:DB_PORT_NUMBER)/MM_DBNAME?charset=utf8mb4,utf8&readTimeout=30s&writeTimeout=30s
and there is one hidden field under "show more settings" where you select the driver, which is defaulted to mysql.
It would be helpful to default to postgres driver and postgres format for the DATASOURCE
postgres://MM_USERNAME:MM_PASSWORD@DB_HOST:DB_PORT_NUMBER/MM_DBNAME?sslmode=disable&connect_timeout=10
Let me know if you need anything from me, but troubleshooting this took a long time and there wasn't a lot of info out there, so I think this would be helpful.