Use SHA as proxy cache KEY

[yamarane]

Problem:

            # For blob requests by digest, do cache, and treat redirects.
        location ~ ^/v2/(.*)/blobs/sha256:(.*) {
            set $docker_proxy_request_type "blob-by-digest";
            include "/etc/nginx/nginx.manifest.common.conf";
        }

        # For manifest requests by digest, do cache, and treat redirects.
        # These are some of the requests that DockerHub will throttle.
        location ~ ^/v2/(.*)/manifests/sha256:(.*) {
            set $docker_proxy_request_type "manifest-by-digest";
            include "/etc/nginx/nginx.manifest.common.conf";
        }

    In these two cases ,  cache key is URI :(`proxy_cache_key   $uri;`) and this treats below 2 blobs as different and consumes extra storage!

    1. my.docker.host/v2/someDockerImage1/blobs/sha256:74924e75d64523f42420d9b0617430fa955e49d835e663e6c838f0ed5209c8da
    2. my.docker.host/v2/someDockerImage2/blobs/sha256:74924e75d64523f42420d9b0617430fa955e49d835e663e6c838f0ed5209c8da

Solution:

Use SHA as KEY

```
 location ~ ^/v2/(.*)/blobs/(.*) {
        set $cache_key $2;
        proxy_pass https://$host;
        proxy_cache cache;
        proxy_cache_key $cache_key;
 ```

[hishamanver]

i definitely think this should be implemented. this allows the cache to use dockers inherent ability to reuse layers