docker-proxy: Deploy a proxy/cache for Docker Hub

Docker Hub's rate limits are so low now that they've started to affect
my home lab.  Deploying a caching proxy and directing all pull requests
through it should prevent exceeding the limit.  It will also help
prevent containers from starting if access to the Internet is down, as
long as their images have been cached recently.

roles/dockerhub-proxy/meta/main.yml

@@ -0,0 +1,4 @@
+dependencies:
+- role: nginx
+  tags:
+  - nginx