Add a nixos module for running the xanthous server in a docker
container, and install it on mugwump including a prometheus scrape
config.
Change-Id: Ifeb315845b7eef2ee33af98fa3f71acdd3d9fe6b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3812
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
