Initialise GKE load balancer healthchecker

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/authentication/infrastructure/-/issues/180

Note that this local repository looks notably different from others as this webapp MUST run in a kubernetes environment to do anything useful. Have therefore foregone any docker compose configuration, in favour of a kubernetes specification targetting minikube. Have removed the usual docker compose based poe commands, in favour of a few helpful minikube versions. Full details on using minikube are on the site as linked in the README - some further explanation of key commands in this README, but have left most of the detail on the minikube site rather than copying it all out!