OK, we are either going to need debian bullseye, or to add instructions to your Dockerfile to install a later version of postgres on top of buster.
If you can upgrade your version of Ruby to 3.0.2 or later, you can change buster to bullseye in the variant and be up and running.
If you want to stay with Ruby 3.0.1, you can follow these instructions to install postgresql 14 on debian buster: PG database migration from Heroku not working - #19 by rjdavies