feat(anubis): add /healthz route to metrics server

Also add health check test for Docker Compose and update documentation
for health checking Anubis with Docker Compose.

Signed-off-by: Xe Iaso <me@xeiaso.net>

test/git-clone/docker-compose.yaml

@@ -12,11 +12,11 @@ services:
   anubis:
     image: ko.local/anubis
     environment:
-      BIND: ":3000"
+      BIND: ":8005"
       TARGET: http://cgit:80
       USE_REMOTE_ADDRESS: "true"
     ports:
-      - 3000:3000
+      - 8005:8005
 
 volumes:
   cgit-data: