Postgres 13 required

[heurekus]

Looks like Guacamole and Postgres 14(+) do not like each other very much. When using postgres:latest in docker-compose.yml, an internal server error occurs. Fix:

Change the following line from:

  postgres:
    image: postgres:latest

to:

  postgres:
    image: postgres:13

This pulls the latest postgres 13.x, which should still get updates until 2025.