add ucam-webauth-proxy image

Created by: rjw57

Add a simple proxy which can restrict access to a resource to anyone with a Raven account or to those within a specific lookup group. This is based on https://github.com/rjw57/ucam-webauth-proxy which is currently being used to protect our Traefik dashboard.

I've set the version number of the build to 0.9 since we've not hammered on it much yet and I'd like the freedom to innovate before a 1.0. That being said, it's deployed already and works so we could bump to 1.0 once we're happy.