Run custom script on postgres docker container after database initialization

To illustrate how to create a custom entrypoint script that'll run PSQL commands every time the container starts, here's what you can do:

Extract the docker-entrypoint.sh script from your image using the command

docker run --rm postgres:alpine cat /usr/local/bin/docker-entrypoint.sh > docker-entrypoint.sh

Edit the file and wrap your commands in the docker_temp_server_start and docker_temp_server_stop functions so Postgres is running but isn't accepting connections from outside the container. Here I've only shown the last bit of the script

                        EOM
                else
                        cat <<-'EOM'

                                PostgreSQL Database directory appears to contain a database; Skipping initialization

                        EOM
                fi

                # run custom commands on container start
                docker_temp_server_start "$@"
                psql -U "$POSTGRES_USER" --list
                docker_temp_server_stop
        fi

        exec "$@"
}

if ! _is_sourced; then
        _main "$@"
fi

Here we run the psql -U "$POSTGRES_USER" --list command which will list the defined databases.

Mark the script as executable using chmod +x docker-entrypoint.sh.

Then you can create a custom image using a Dockerfile like this

FROM postgres:alpine
COPY docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh

Now, every time you run your custom Postgres image, it'll list the defined databases.

To run migrations, you'll need to expand the code to figure out what needs to be run.