You are correct. There's no direct way to edit master.cf via environment variables. This is because in most of the cases, the file does not need to be changed and because editing via postconf is more complex.

However, there are two options how you can still do it with this image:

1. mount master.cf file and supply your own copy, or
2. Supply your own init script in /docker-init.db/ directory which will run the necessary postconf commands.

If you would like to add this functionality, I suggest looking at how postconf is used to edit master.cf.

This image uses postconf internally to do manipulation of main.cf as well.