/etc/postgresql/12/main/postgresql.conf is where you define the interfaces for postgresql to listen on. It’s done with the line “listen_interfaces =”
So, to get postgresql to listen on a docker interface, you have to add the IP address to the configuration:

listen_addresses = 'localhost,172.18.0.1'

This will make postgresql listen after connections from my docker container. Unfortunately when i rebooted the machine, postgresql was only listening on the localhost. That was strange.

After doing some fiddling around, I discovered that I might had a timing issue, and I even found a couple of articles covering the issue. The postgresql service needs to wait for the docker service to be started, or it cannot listen on the docker interface, that seems logical. I tried following some of the workarounds described, but I couldn’t get it to work. I then took a look on how the default systemd is build, and it seems that /etc/systemd/system/default.target.wants/ consist of symbolic links to /lib/systemd/system/whatever, so i did the same:

sudo ln -s /lib/systemd/system/postgresql@.service /etc/systemd/system/default.target.wants/

I then added this line to the unit block:

After=docker.service

This seems to have done the trick. After a reboot, postgresql is listening on my docker interface. You can always verify with:

sudo ss -nltp | grep postgres