WIP: Postfixadmin config

This is supposed to fix 2 problems:

• setup password is not working in my experience, partly because the config is overwritten after container restarts
• password check does not make sense (https://gitlab.e.foundation/e/infra/docker-postfixadmin/issues/1)

For the first, according to Docker hub description, it should be enough to specify POSTFIXADMIN_SETUP_PASSWORD as environment variable. But this is not working.

For the second, we need to use our own config. I tried to mount a custom config.local.php as a docker volume, but this gets overwritten with default values after every restart. The docker hub description also mentions mounting a custom config.local.php, but the path mentioned there does not exist.

Please merge !23 (merged) before merging this.