3-tier implementation of manifest caching (#57)

* implement manifest caching; refactor config with includes, and generate from ENVs in entrypoint.sh
- disabled by default; enable with -e ENABLE_MANIFEST_CACHE=true
- default times and regexes are a wild guess, make sure to tune for your use case.
-  add manifest caching/anti-ratelimit usage note to README
- add -e ENABLE_MANIFEST_CACHE=true to examples, some wording changes
- add -e ENABLE_MANIFEST_CACHE=true to one the steps in test workflow.

nginx.manifest.common.conf

@@ -0,0 +1,8 @@
+    # nginx config fragment included in every manifest-related location{} block.
+    add_header X-Docker-Registry-Proxy-Cache-Upstream-Status "$upstream_cache_status";
+    add_header X-Docker-Registry-Proxy-Cache-Type "$docker_proxy_request_type";
+    proxy_pass https://$targetHost;
+    proxy_cache cache;
+    proxy_cache_key   $uri;
+    proxy_intercept_errors on;
+    error_page 301 302 307 = @handle_redirects;