Hi Alexander and welcome to the Mattermost forums!
I’ve never seen these specific error messages, so I’m not 100% sure what exactly the problem is.
As you can see in the .env file, the variable MM_SQLSETTINGS_DATASOURCE is generated with the values of the other variables and if your docker-compose version is too old, it will not support variable substitution inside the .env file
To verify that this is your problem you could modify the .env file to not use the variables but the values instead:
MM_SQLSETTINGS_DATASOURCE=postgres:/mydbuser:mydbpw1234@postgres:5432/mattermost?sslmode=disable&connect_timeout=10
If this works, then my assumption is correct and you would just need to update your docker-compose binary.