Nginx security improvement

[kramos]

I ran gixy on ADOP: https://github.com/yandex/gixy

It seems that adop-nginx/resources/configuration/sites-available/registry.conf has an area for improvement:

====================` Results ===================

>> Problem: [host_spoofing] The proxied Host header may be spoofed.
Description: In most cases "$host" variable are more appropriate, just use it.
Additional info: https://github.com/yandex/gixy/blob/master/docs/en/plugins/hostspoofing.md
Pseudo config:

include /etc/nginx/sites-enabled/registry.conf;

        server {
                server_name ~^registry*;

                location /v2/ {
                        proxy_set_header Host $http_host;
                }
        }

==================== Summary ===================
Total issues:
    Unspecified: 0
    Low: 0
    Medium: 1
    High: 0

I changed:

adop-nginx/resources/configuration/sites-available/registry.conf

Line 41 in fc94e26

proxy_set_header Host $http_host;

to

proxy_set_header    Host                $host;

And it made gixy happy.

I wasn't sure how to test the docker registry with this change. Happy to test it and do a PR if anyone can suggest some test cases.