I would like to be able to hook in just before PostgreSQL starts and add my own script there.
I would use it to add a script that reads environment variables that start with PGCONF_ and add them to the startup parameters.
example:
docker run -e PGCONF_log_checkpoints=on postgres

This would start postgres with argument log_checkpoints=on

I already use this for a while, but I needed to do some sed in the init script to make it possible. I would be happy to contribute a PR, but I want to make sure this is something that can be accepted.
I replaced the last line with this script:
https://github.com/xenit-eu/docker-postgres/blob/master/src/main/docker/before-startup.sh
Our images are published on docker hub:
xeniteu/postgres