Add possibilty to configure database port when using docker image

It would be really nice to have an environment variable for controlling the database port when using the docker image. I thought I could just add the port to the host string, but that leads to something like this inside the hibernate.cfg.xml connection url: 

jdbc:postgresql://localhost:9947:5432/xwiki?

I need to fully automate the deploy, so it is not a good option for me to edit the config file manually.