introduce env DEBUG_HUB; to help understand #54

- injects a mitmproxy between the caching layer and DockerHub's registry-1.docker.io
- it also disables VERIFY_SSL so the mitmproxy can be used;
- DEBUG_HUB's mitmweb is exposed on port 8082
- both DEBUG and DEBUG_HUB can be enabled independently

nginx.conf

@@ -81,6 +81,7 @@ http {
     # Just in case you want to rewrite some hosts. Default maps directly.
     map $host $targetHost {
         hostnames;
+        include /etc/nginx/docker.targetHost.map;
         default $host;
     }