fix: make host.docker.internal reference work on Linux

[iwilltry42]

host.docker.internal is not necessarily present on all systems (e.g. not on my Linux machine - it's there by default in Docker Desktop though), but it is used as a default connection string to Llamafile by LiteLLM, so we can just "hardcode" it in the docker-compose file. host-gateway is a special DNS key that will be replaced with Docker's host gateway IP and written to /etc/hosts inside the container.

Please test this on Mac or Win with Docker Desktop to ensure it doesn't break those.